One of my campers came with dexter torflex axles and a genuine 4" channel frame. It just needed real crossties and decking to make it a real trailer. Another one was made of 4" stringer channel and all stamped gauge crossmembers.
Moral of the story, you gotta crawl under each one.
Most of them, i suggest doubling up under the existing frame with a second piece of channel that runs from hitch to forward leaf spring eye. Thats where the bulk of the load is on a car hauler.
God Bless America