I've built 2 utility trailers from travel trailers. One from a 19' Coachman with a 5,000 lb GVWR, the other from a pop-up with a 2,000 lb rating. cross members are the weak point of these frames as they're constructed for floor decking, which increases o/a strength. I added additional crossmembers and integrated a side rail to strengthen the whole unit.
The 19' unit I shortened to 15', added a rail for ramps and a winch for loading non running mowers, etc. This trailer has electric brakes on both axles. Works great. Down side is that most travel trailers have narrower wheel width. So, unless you build a "deck over" the tires, you're not going to get a very wide trailer decking.
The Pop up is a little over 8' long on the deck. Just completed this project. "re-fabrication" technique the same as the Coachman.
Remember to install new tires as the ones on the thing are probably dry-rotted. I'll send pics later today .
Good Luck, and don't worry about carrying the rated load, IF, strengthened properly.