Great idea, getting out and seeing the country before the little ones get sick of the parents :)
Ditto all the feedback above. Way too much miles/day, IMO. I have kids nearly the same age. The most we ever travel is 5 hours and that is a LONG day, 3-4 is our target. And we usually travel 1/2 our days, max. Even then, the kids wouldn't mind a little less traveling.
Renting is my advice too. You should be able to get away with 200/night x40, it's only 8,000. It will include breakdown insurance, and if something goes wrong, it's the rental agency's problem. I'd expect the whole buy/sell transaction will cost that or more, especially if you want to get rid of it fast, and not store it for months while waiting for it to sell.
