I think you're in a VERY tentative position with your budget and you need to proceed VERY carefully. First, older high end coaches are fine if you had 75K-100K and some savings for repair. Buying a 27 year old Prevost bus would be a recipe for disaster. The first major repair, and at 27 years old it's coming, would probably wipe you out financially.
Buying a coach at a 20K price may get you around for awhile, but it's going to cost you to keep it running. Many will jump in and say their 20K coach runs all over the country and that's great, but betting on it running flawlessly is a gamble.
Only you can really decide what you need and how you'll use it. From what you describe as your situation, I would buy a good quality 2-3 year diesel 3/4 or 1 ton truck and then use a smaller budget to buy a decent trailer or fifth wheel. You'll at least know you have a reliable vehicle and the worse that could happen to the trailer, that would prevent you from moving around, is that the axles fall off. Then if you find that you really like the RV lifestyle, wait awhile and step up to a nicer trailer.
