A possible source of trailers is a trailer park. Sometimes people abandon wrecks and they get hauled into the bush.
For the record, RV frames only work well in conversions to other things *if* you make sure the decking you attach is secured in a million places to add torsional rigidity. On their own, they're waaaay flexible usually as they rely on the structure of the trailer (that's no longer there) as some of that rigidity.
And yes, you'll end up with a leaf-over (rides high) axle that is probably 3500# rated (assuming a single axle trailer) that will likely ride like a tank unless you re-do the springs.
After you pay to demo the trailer and rework everything they're seldom worth the effort in the end.