So far it appears that no one is pushing the Prius. Ok...we'll rule that out. With just us 4, a van would be comfortable but so would the mid size car for better gas mileage and a better price. The truck is still highly considered, but the wear and tear, etc. would be a bummer and we would probably need to replace the tires, which tacks on a lot more money. My DH did a spreadsheet, and with hotel, gas, rental fees, etc. as part of the formula, the best net cost is the mid size car. The van is very expensive and the truck is pricey with the gas.
I would still like to research buying one way tickets and driving back, but it seems cheaper to return the car back to where you rented it from. Is that entirely true?? 
Our itinerary is to drive from Sacramento to a little past Tuscon. Then the next morning head to San Antonio, then New Orleans and then Orlando. This would take 4 days. Then relax in Orlando with our DVC points, then take a 4 day DCL, then return to WDW to hit the parks for another 4 or 5 days and then return back home. The drive home is what I am most worried about!!!
Hi Couple of things,
First given you're itinerary at WDW, you may want to reconsider this trip. Because given things you've already said, You WON'T enjoy it.
You're looking at 2 twelve and half hour days, an 8 hour day and a 9.5 hour day, that's
without any stops. Factoring in traffic, gas, food and potty breaks you're looking at two 15 hour days and two 12 hour days, which allows you absolute no time to see anything but the highway.
I'm basing my numbers on the following figures from
AAA:
Day 1 - Sacramento to Tucson - 868 miles, 12 hours 21 minutes driving time.
Day 2 - Tucson to San Antonio - 868 miles, 12 hours 21 minutes driving time.
Day 3 - San Antonio to New Orleans - 543 miles, 8 hours driving time
Day 4 - New Orleans to WDW (I picked AK as it was closest) - 646 miles, 9 hours 23 minutes driving time.
Second, You say you'll have to get new tires on the Tundra, truthfully, I wouldn't factor that into the cost of the trip as that's a maintenance expense, something you'll have to do anyway. Yes, you'd have to do it a little sooner, but no matter what, unless you sell the truck, you're going to have to change those tires.
Good Luck with your decision.