I would first of all take it to an RV refrigeration repairman and see if there are any recalls that need to be done. Also, when they sit for periods of time, spider webs can get in the gas line tubes, etc. If it's leaking ammonia, forget all that and look at replacing the cooling unit. But if it's not smelly and you can't see any yellow gunk when you take the panel off, I would start with a tune up.
If you need to replace the cooling unit, we got ours here and it's running great. . .
Fix your FridgeDale