I just spent 2 months reviewing and pondering enclosed trailers. I finally (and only recently) went with CargoMate (factory in ?Salem, dealer in Olympia) for price and budget. It's not a high end by any means, but it did compare on the key fundamentals. 3/4" floor, 3/8" walls, dexter axles, etc. The undercoating is a thick rubber and is impressive. (minus the touch up spots they missed, but I signed up for steel and I now have a can of rustoleum and undercoating as general maintenance.) They really skimped on the finishing work (which was ok with me), like the moulding and the roof vent is just flat cheap.
What finally sold me was seeing ones around that are older and still holding up. I spent more time stopping and inspecting other peoples trailers than listening to the sales people. Charmac is supposed to be quality, but on one dealer lot a new one had a roof leak and a 1 year used one had surface rust on the steel frame and they were $2,500 more.
My storage lot has a CargoMate and Interstate in the mid 2000 years. They seem to only have minor rust in typical wear spots and more importantly, not underneath.