In 2010 I bought a 1998 Roadtrek 190 (see photo) with 94,000 miles for $20,000. In excellent condition. Over the years I have replaced tires and transmission. Great class B. I also, at the time, paid a 10% finders fee as I knew nothing about motor homes or where to look. I live in N. CA and the finder located this RT in Oregon and traveled up there and had it checked out mechanically. The $2,000 finders fee was well worth it.
Don't be in a rush and look at as many as you can locate and get an idea as to how they shape up at $20,000. Since I got mine almost 10 years ago for $20,000 I would think you should find an oldy for that money in very good condition. Mileage preferred is very subjective as there are many variables. Road test a few and get a feel for what $20,000 will get.
Good luck, Bob