I know this is a year old post, but here is my findings after a few weeks of research into pre-owned trailers.
There is a ton of units out there for under $20k that are super nice and hardly used. If I were in the market now, I would have a large selection of fivers ranging from 25' - 35' in length, some toy haulers. Since my wife and I will be new to camping ourselves a nice used unit is in the works. I found a sweet Forest River 26RL 30' fiver for $17000 that if my wife and I were in the market, we would definitely go check out.
Class A motorhomes I didn't look into because wife and I want to have the truck with us to run errands, etc.. and not have to unhook the camper. I have an F350 CC/LB truck and she has an AWD Edge so flat towing is out of the question. Not that one is not in the cards, but we would like to get a Toy Hauler so we can take my Harley with us as well.
A good diesel truck can definitely be had for around $20k, but you may have to put in some extra money for upgrades/replacement parts. I work for a diesel performance shop part time and the boss buys/sells trucks all the time around that price range. Most recently, he sold a 2005 F250 King Ranch diesel with 135k on it, newer bottom end, EGR delete, new exhaust and new rear axle for $19k. Truck's interior was mint and definitely didn't look like it had that mileage.
OP, hope you bought something in the past year!
2013 F350 CC/LB 6.7 Powerstroke - full deletes, 20x10 Fuel's, 35X12.50 Toyo's, LED lighting
Former RV's - Coachman Leprachaun, Coachman PathFinder, Holiday Rambler Imperial, Coachman Catalina