Our camper has an added deadbolt. It was done long before we bought it. It is a regular Schlage deadbolt that one would buy at Home Depot. A slot was cut in the door frame for the bolt to slide into. While it is true that the door is flimsy, I feel better with the deadbolt locked and the door is less likely to open. Since we have a truck camper the frame bands and twists a lot and I like knowing that the door won't pop open.
A locksmith could have one installed properly and quickly for not much money I would imagine.