Was with a friend of my mom's who had a Coleman popup trailer. Very simple compared to today's rigs, just a glorified power strip for internal power, maybe a gas stove, a small water tank and city water pressure hookup with a manual pump faucet, drain going directly out the rig. Managed to sleep in it in sub-freezing temperatures with no heater at all. Brrr...
Funny thing was how well engineered that popup was. The sliding bed rails were permanently attached. The door could be locked by folding the step up and locking it, or just pulling a little bit of rubber gasket out to flip the latch and pressing it back into place... good enough to keep honest people honest.
Too bad Coleman popups are not being made anymore... just the little things made it a step ahead of the Jaycos and Starcrafts at the time.