Most likely doable for $20K. Huge expense after purchase could be tires if date code is over 5-7 years. Then batteries, refer, AC are other expensive single items on the RV parts. Power train on the chassis, with engine, transmission, AC, and suspension. But start getting up to speed with better brands, and not the flash and mirrors. Some of the newer stuff have lots of wide open interiors, and not storage. Not a problem till you start to load to go.
Would suggest watching this web site daily, and see the way prices are always moving down to your target. This one company reports selling about 150 units a month.
http://www.pplmotorhomes.com/
Have fun.