Oh, my! Did I write this post? We are also planning a cross-country driving trip for 2-3 years from now! We're starting from the East coast and our plan is to go to California and back in 4-5 weeks. As a matter of fact, my very rough-draft plan is laying on the desk in front of me right now. Here are the decisions we've made thusfar:
We started with the idea of renting an RV or a pop-up of some type, but after some investigation, we dropped the idea and decided to stay in hotels. We are a camping family, but we decided this just wasn't the best decision for us. Why? You'll make more miles each day in a regular car, and we want to maximize time having fun rather than time driving. We have plans to visit many big cities as well as National Parks; we can't see ourselves driving a big RV through downtown Chicago, nor do we want to have to back-track to the campground to pick up a pop-up. Next, making and breaking camp takes a significant amount of time each day. We plan to make our trip in July, and the lack of air conditioning was a consideration. If we've hiked all day, we want to come back to a cool room. We also knew that some nights we'd be setting up camp after dark, which didn't appeal to us. Don't forget, too, that camping isn't free. We usually pay $12-20 (to tent camp) per night, depending upon location and season -- that could be 1/3 the price of a cheap hotel. The gas savings (from not pulling the pop-up) will probably bring you to 50% the cost of the hotel room. Will you need to buy any equipment to camp (lanterns, propane cookstove)? If so, don't forget to count that in the cost of your trip. I don't know if you will reach the same decision, but we decided that though they'll cost a bit more, hotels were more realistic for us. We're a family of four, so we fit into a standard hotel room nicely.
You asked about KOA campgrounds: I've only stayed in one (Savannah, GA). We rented a "cabin" -- it looked remarkably like the storage shed in my back yard except for the bunk beds inside. You know, a little barn shaped thing made of cheap wood. We were awakened during the night by the police chasing a criminal through the campground. Realistically, I know that I could be at the Ritz-Carlton, and the police could run through -- but it gave me a bad impression of KOA, and I won't be back. It was one of the worst campgrounds I've ever stayed in.
We really like the State and National park campsites. Typically you pull in and drive through the campground until you find an empty site that suits you. Prime sites have level ground and are close to the bath house. If there's a "loop", the campsites on the outside are better spots (bigger, more privacy). Sometimes they have one section for tent campers and one section for RVs and pop-ups (I think that's because of the need for electricity/water in the RVs). Campsites vary: You can count on having a fire ring and a picnic table; you may also have a lantern hook, an electrical outlet, or a fire extinguisher -- it's always a surprise! The bath houses are usually decently clean, but they sometimes run out of toilet paper. You may have to wait in line for the shower. When you arrive in camp, you set up your equipment and the ranger comes around later (usually at dinner time because that's when everyone's "home") and collects the night's fee. Most campground people are very friendly: you see kids riding their bikes up and down the road, everyone speaks as they walk about camp, etc. Lots of people bring pets. I've found that most of the time campers aren't "going somewhere" as you will be on your trip -- for most of them, camping IS their destination. Most of the time, everyone's quiet by 9 or 10:00 in the evening. Often the rangers offer very informative nature programs (free or very low cost), which we enjoy very much. Epinions.com is a good place to glean information on campsites and National Parks.
I wholeheartedly agree with what someone else said: Since you are not a camping family, you should really try a couple trips before you commit to this lifestyle for a month or more! You need to be sure that you're okay with the walk to the bath house, cooking in the pop-up, etc. And it might be the right choice for your family -- I'm sure we don't have identical expectations for our two trips! I don't mean to sound like I"m trying to talk you out of camping -- I'm just telling you how we talked ourselves out of it!
We chose Days Inn as our "hotel of choice". It's everywhere, it's cheap, and by using the same chain regularly, we'll earn reward points. Right now their deal is good: stay twice, get a $25 gift card. For this trip, we cannot afford luxury, and we're fine with that. We're the type of people who can go to a fine oceanfront hotel, then go camping the next weekend -- we're very flexible and we adjust our expectations according to the event.
We have also found a couple Hostels (they've dropped the name "Youth Hostels" and they accept people of all ages now) that look pretty good. Both DH and I stayed in Hostels as teens, so we have a positive attitude towards them. Typically, you pay about $18/person/night to stay in one. You can get a family room, or you can split up into same-sex dorms. The price includes linens, a bath down the hall, a common room with TV, pool tables, etc., and a kitchen -- it will vary from location to location -- some hostels include a free breakfast. One big benefit: hostels almost always have washers and dryers. They typically do not have hotel-ammenities like maid service or pools. Do a google search ("hostels + Philadelphia"); I found one hostel in St.Louis that sounds like a dump, but the other two I've located (Philadelphia and Chicago) sound like places we'd enjoy staying; both are right in the middle of the things we want to see and are bargain prices compared to the hotel prices in these big cities! I also found one in Orlando that will be perfect for my scout troop when they go to Discovery Cove.
We're also checking out some cabins in the National Parks; they'll be more expensive, but they make sense for some locations. We'll probably enjoy a bit more space as a splurge on those nights.
We don't have a complete budget for our trip yet, but we're putting money aside each month into a special account. We're very frugal people, and we will not go into debt for this trip. If we have to put it off another year to save enough, that's fine. If we do it in 2008 (our target year), our girls will be 12 and 15, and we think that's a perfect age.
We're planning one restaurant-meal per day, and the others will be picnic-type meals. We'll take a small grill, a cooler, etc. We'll bring many refillable water bottles; I took a trip like this when I was a teen, and I remember specifically that our bodies craved water-water-water in the dry heat of the desserts and plains.
We're definitely going to buy an Eagle Pass (I think that's the name) for the National Parks. It gives you and everyone in your car free admission to most National Parks for one calendar year. Cost: $50, which will pay for itself the third time it's used! Because we have many National Parks on our agenda, this is a no-brainer for us. This pass also gives you a discount on some ranger-led programs and discounts in some gift shops.
The park service also sells a "Passport to Your National Parks", which will be in everyone's 2006 Christmas stocking. It looks like an American International Passport, but it has a place for a stamp for every National Park and Monument -- I think the kids will get a kick out of this. See them at eparks.com
Some of the things we plan to do are very expensive (a hot air balloon ride near Mt. Rushmore and the Crazy Horse monument), others are free (the St. Louis Zoo and hiking to the Continental Divide in Rocky Mountain State Park). Adhering to a "scrimp and splurge" theory, we're making no attempt to be consistant in our spending. We're choosing the things that we really want to do, and we're going to do them regardless of the cost -- we may never return to some of these places again; on the other hand, when a good budget alternative exists, we're going to take it. We know that the low-cost activities (i.e., free museums) will be repeated many times during the trip; the expensive activities (i.e., white water rafting) will be one-shot deals.
We're finding that a few things need to be reserved FAR in advance. For example, if you want to do a mule trek to the bottom of the Grand Canyon in the summer, you need to make reservations something like 20 months in advance. So we're not really all that far ahead!
We plan to give the children good hiking boots, good day packs, water bottles, etc. on gift-giving occasions as the trip approaches. The Christmas before, I'm going to give them each a $150 pre-paid VISA for their souveniers; that'll be a major gift for them. From previous vacations, they understand the "I am responsible for my own souveniers and when the money is gone, it's gone" theory.
Once the time is closer, and our route is planned, we'll go to restaurant.com and buy some coupons; you can always get a code from mousesavers.com at the end of the month, and you can often buy $25 gift certificates for restaurants for $3-5 (read the details on every gift certificate carefully -- some of them limit days of the week, etc.). DH and I will also try to collect gift certificates for fast food, etc. as the time draws nearer; we always seem to have them around, so we'll save them up for the trip.
DH did a computer search for travel and tourism for every state in the Continental US. The mailman hated us for a month -- we got TONS of stuff from every state. Some days it literally wouldn't fit in the mailbox, and he'd come by and -- WHAP! -- throw it at our metal door. The kids loved reading the booklets. Perhaps most importantly, many states sent us actual, full-sized maps for free. Warning: Do not request anything from South Carolina. They killed a whole forest mailing us stuff. The irony: We live in North Carolina, and we're not planning to include the states adjacent to our own in the trip activities -- those places are all do-able as long weekends. We want to focus on the farther states. We also decided to skip Washington DC because it could easily take a whole week -- that'll be another trip for us. And we eliminated Florida from our planning because we've been all over Florida so many times.
Our organization methods: We have a notebook with a page protector for each state, and when we find information about something we want to do in that state, we slide it into that protector (we have cut up all those brochures, and we have kept only the things that interest us most). We have a tenative plan now, so we'll be eliminating a few states and weeding out the activities that don't make the cut. I plan to eventually have two notebooks to take on the trip: 1) An activities notebook which will contain our route, brochures for places we plan to visit, hours of operation, notes about "must sees", tickets that we may have bought in advance, etc. It'll be laid out as Day 1, Day 2, etc. 2) An overnight notebook, which will have a page for each place we plan to spend the night. Each night's page will contain the name of the hotel, its address and phone, a confirmation number, driving directions, etc.
Whew. That's what comes to mind first -- random thoughts at best. Please feel free to ask about other details we're planning, and I'd be interested in hearing your plans too.
You also need to look at your insurance and make sure it covers your friends' pop-up.
they often have free nature programs, too. Our family has been camping for many years, only going to a pop-up on the last 2 yrs. I wish we had bought one earlier, it is so much fun. But I'm pretty sure i would not want to go cross country with it. 
