Class Bs are great, but very small - limited storage, refrigeration, and holding tanks, so not really suited for parking to use as an apartment for weeks at a time.
Bs are also in great demand by people who want to travel to see the country, so prices are high. $20,000 might get you an old, well-used, high miles, gas van that would need some repair work and regular maintenance.
Finally, RVs are great for do-it-yourself types who tinker with one sitting in the driveway....I don't want to throw cold water on your plan, but people who are not mechanically inclined are probably not the best candidates to enjoy RV ownership. Things always break down or go wrong, and if you have to pay somebody else and wait around for them to order parts and fix stuff, it will get really tedious and expensive fast to own an old RV, especially if it sits most of the year.