Yes Ammonia is the cooling agent. However, if you don't smell leaking ammonia, the repair cost could be minor. If the ammonia is leaking it's a throw out. (I'd put it out on big trash day with the doors off, and just label it "RV refrigerator, ammonia coolant all leaked out").
Could be something as simple as a fuse. Or maybe a circuit board. Many of these old refers will last for 30+ years. If you just don't want it around, why not give it away on the classifieds. Someone will pick it up and fix it.
Today is a gift, that's why they call it the present.