I believe there are plenty of late 90's motor homes that have plenty of life left in them. I have one, a 24' 1997 Coachmen with 56,000 miles , that I'll be selling whenever I can get my son to paint it (I've been waiting for over a year now :R)
There are tons of motor homes still running around that are much older than that. For your 20-30K you should be able to get something a bunch of years newer than late 90's. I agree with what was posted earlier that a late 90's, in good shape, should in most cases be in the 10-15K range, maybe less.