OP, In NPs there are roads, especially in Yellowstone that are restricted to 20' or less vehicles... cars/PU so even a short Class C is prohibited. Buy a BIG MH and tow something that will handle your family for sightseeing. Do not make the mistake I made of listening to those that say 36' is the biggest for NPs. I bought a used 36' DP and it did not take very long to realize that is not true. We were in the Tetons and Yellowstone this summer and stayed in three different USFS CG's in our 42' without reservations in late June.
There is not much between NY and WY worth seeing compared to WY and MT so 6 hour days will be a waste of time that you should be spending in the West on a three week trip. While I do not suggest you follow my example this summer of N GA to Jackson Hole in 48 hours... the two best ways to see Kansas or Nebraska is at night or from 30,000'.
