Refrigerator Issue

I have a 1996 Sunline Solaris with the normal Dometic Gas/elect Fridge. Last summer it slowly started giving up the ghost on cooling. The failure was not sudden but took place over the summer until we had no cooling at all. We have ignition although I have to manually light it and it's been fine for years. Since the failure was slow moving I suspected the refrigerant, perhaps its gone after 20 years. I can not detect any ammonia order at all so if there is a leak its very small. Is it possible to recharge these units like a car AC unit??

BTW I run it only on propane.

Thanks for your help.