Under $20,000 is going to mean mechanical work at some level. Even if the RV was well-maintained mechanically, things get old and wear out. If you buy a class A and can't do mechanical work yourself, you're going to lose your house every time it needs mechanical work. In addition, you won't know when or where it's going to breakdown, so boondocking is a risk to be factored in.
While I LOVE my 40 year old clipper, if I were going to full-time I would buy something 2-5 years old regardless of cost.
Just as an example, last summer I had ignition issues that could not be singled out. The clipper spent a couple months at the mechanic's while he kept trying to get it to repeat the problem in order to diagnose it. Never could get it to do it at the shop, but as soon as I took it out on the road, it repeated. (Ended up doing a mod to bypass the Mopar ignition system completely - now works like a charm). I would have been homeless during that time if I was full-timing.
You're probably better off going with a travel trailer and a tow vehicle. That way you can park the trailer and stay awhile if the engine needs work. And likewise, if you have a problem with the trailer, you can still get around in the vehicle while it's getting fixed.