Ammonia evaporation refrigerators have a fairly defined lifetime. the maintenance protocol is to do the best you can with with you have. Keeping the pipe clean and free of rust which you can do from the outside; remembering that nasty bee hives can plant themselves near LP gas is important. You can replace the ammonia evaporation coils without having to replace the entire frig or taking it out of the camper. Usually that fixes the problem. There used to be several companies making camper refrigerators; now just one or two. You can order the coil units from just about any camper dealership. Make sure you have the exact model number. They take the old one back and send it back to the manufacturer.