Is storing bikes in an enclosed trailer a good idea?
So long as the trailer is reasonably weather-tight it shouldn't be a problem.  Since there's nothing in the trailer generating humidity, no significant condensation will occur, and, within reason, temperatures won't hurt bikes.  (The only caveat would be that you maybe should have some vents that can be opened if temps rise above 90F or so, so that temps don't get over about 150F inside.)