Lots of good data here. The only thing I will add is this. Don't buy too small. Most go for a class C because they think the size makes them easier to drive...(WRONG) I bought a 22ft class A for my first RV and towing was a joke 454 with 24ft box trailer and 4 to 6 MPG and the thing was working overtime all the time. I bought it because I figured small enough to move around easy.
My current on is a 1995 33ft Monaco DP, tows likw a dream and yes it could be faster up mountains but who cares i'll go fast when I get there. I get between 9 and 11 MPG most always and have seen a little higher if I drive conservative. I walked in on the right day and made the right deal and drive out under 50K. The only negative so far is the LP generator but I am planning on changing that over to diesel next spring. Also remember there are other benefits to owning a coach good financing and you can write them off as a second home.