I used to buy trailers and motorhomes that were in a little distress, fix them up, and flip them for a few dollars profit, once upon a time. You really need to understand what you're looking at for an RV, whether it's a motorhome or trailer before you make an offer though. The trailers I'd buy and sell were either Airstreams or the little fiberglass egg trailers. I had a ready market, and even if they needed a new axle or some other major repair, I knew that they'd likely be structurally sound.
Know what you're looking at when you see an RV. Spot wood vs. steel vs. aluminum framing. There's nothing necessarily wrong with wood, depending on what he outer covering is and how it's applied. My Super-C motorhome is framed in wood, but it also has three roll bars in the coach that help support the frame, and there's no seams in the fiberglass roof to leak. That doesn't mean it can't leak around through-hulls and windows though.
Learn to spot dry rot and where it forms. Look at the frame. Check out the appliances. You might find a coach that is significantly discounted from market value from a private party because one or more of the appliances need to be replaced. Know what those costs are and labor costs if you're not willing to do it yourself so you can decide if it's still a good buy after repairs.
You might occasionally find a good deal at a dealership, but they're in the business of selling RVs, not making sure that you get the best RV for your money. On a used coach, they're not going to fix anything you don't point out, unless it's something safety-related. Any "good deals" on a dealership lot are going to be those RVs that have been on the lot for more than 120 days and get little 'looking' action. Occasionally, if you're looking for a specific coach or trailer that is undervalued in the market but that has the features you want, you may be able to find a deal at a dealer. I bought my Born Free Super-C motorhome that way from a dealership. People just weren't interested in it because they didn't know what it was. I, on the other hand, knew EXACTLY what it was and put the deal together within days of finding it. I bought it about $20k under market value.
Your best deals, though, are going to be on Craigslist. And don't be shy about stopping at people's houses if it looks like there's something you might be interested in sitting in the yard unused.
RVs and trailers in particular are pretty simple gizmos, for the most part. Once you know what to look for, what makes one more desirable over another, and how to determine if it's a quality coach in good condition, it really makes no difference whether you buy from a dealer or a private party. You'll know you're getting a good deal on a trailer in good condition, you'll know what it needs, and you'll know what all that's going to cost you.
Happy hunting!