I'll chime in on camping with used RVs as it's something we've always done. We started with what I'll call "classic" rigs about ten years ago. Our first was a 1973 22-foot fifth-wheel (couldn't even tell what brand it was) that we paid $800Cdn for and after a couple of years "upgraded" to a 1974 28-foot Prowler fifth-wheel for $1500Cdn. We're now into the more modern rig in my signature. But running those classic rigs taught us a lot and we were able to get out and half fun for not much money.
Here's what we had to look at and look after. The biggest thing was the brake wiring. A dealer might check things like this out but get under there and look and test before you get too far once it's hooked up. I rewired both the old trailers as the brake wiring was almost non-existent. Another thing we had to do on both those oldies was replace the refridgerater. A new RV fridge costs more than those two trailers were worth so we removed them and dropped in an inexpensive bar fridge from Walmart. So as long as we were hooked up to power we had an operating fridge, which was fine as that's what we always do anyway. There's a chance you might also have to replace expired propane tanks if the trailer hasn't been used in a while. And of course keep an eye on the water lines. Even if it was properly winterized, fittings can eventually fail and leak when they get old.
So older trailers are certainly worth a look but prepared to do a little work before you head out!