We had the same problem. We used two solutions.
1) We got some 2 or 3″ wide self adhesive velcro and placed two straps at the edge of the door.
2) We placed a bungie diagonally across the door from a furnace vent to the roof lift bracing.
I can send you a photo if you are interested. Camper is stored across town so next time I am there I can shoot a photo.
I think the cause of this problem is that when the camper is at a certain configuration, the latch on the fridge does not actually engage. This can also be true if you put a lot of heavy product in the door, such as a bunch of beer.
We had to take the camper off the truck at one location and camp in it. We found that the change in weight distribution to cause the fridge latch to engage in a very strong and secure way. My suspicion is that it would be possible to shim the fridge that would cause the latch to engage.