I did some basic research into renting an RV for DD and myself to go cross-country next summer and the prices, when I added in gas and campgrounds, was crazy

It will be MUCH cheaper for us to take Amtrak, stay in 4* hotels and eat in restaurants for the three weeks.
We just took a cross-country driving trip last summer (it was absolutely WONDERFUL, and I hope you enjoy yours as much as we did!).
We started planning and saving for this trip a couple years ago, and we chose 2009 as "the year" because we thought our kids'd be just the right age. When we started talking about it, we began with the assumption that we'd rent an RV. We thought it would be easy because we'd only pack/unpack once, and we thought it'd be a budget choice. WRONG!
When our chosen dates were about a year away, we started to get serious about our planning -- the route, etc. -- and we were shocked to learn just how expensive an RV is to rent! It was going to cost well over $1000/week, and that didn't include mileage fees and cleaning fees for the rental company! On top of that, the gas mileage would've been horrible, and we'd still have needed to pay for campsites. In addition, there were some non-financial issues: Campsites tend to be somewhat outside town, whereas hotels are located right next to the things we wanted to see. Also, though our trip centered on national parks, we also spent some time in the big cities, and neither of us wanted to pilot "a house" through Denver or Las Vegas.
Instead, we stayed at moderate hotels (lots of Holiday Inns and Best Westerns) and cabins at the national parks. We used our credit card points, we used Priceline, we compared every option for every stop. I'm a number-cruncher, and I figured up that we spent
less than half using hotels and restaurants. Traveling with the kids, 2/3 of the time we had a suite or two connecting hotel rooms. I'd recommend this route to anyone doing a similar trip.
In defense of RVs, it's a lifestyle choice. If you love the idea of quasi-camping, of having a campfire and being close to nature but still sleeping in a bed, you might be willing to pay the price. I suppose it's not bad cost-wise if you OWN it and USE IT repeatedly (or if you could borrow or barter for its use), but renting is just crazy expensive. I think it's also a better choice for people who want to drive to one location and STAY THERE; for our trip, which spanned more than 3000 miles, it would've been a poor financial choice.