At least one was bought from a dump...
I do not know if you guys are familiar with Homemade Home, but he buys houses for $5,000 - $15,000, spends at least six months renovating them, and then rents them out. He cut the walls and roof of this twenty-foot trailer in half and pulled them apart, ground off the rust, sprayed rust converter, and then Rustoleum Rusty Metal Primer. He welded in reinforcements, put the aluminum bottom back, and then bolted in floor boards, and screwed in plywood:
This guy (with 770 views) stripped a 24-foot-fifth wheel until it was just the frame and the roof. Then he attached a rope to it and pulled until the roof came down. He said he spent weeks separating metal from wood and received $200 for it "which wasn't worth it at all." He did not say how much the trailer was, but he said the entire project cost $5,000: