My last one lasted 13 years before the cooling tubes corroded and leaked ammonia. My present one (refurbished) lasted 3 years before the thermostat on the electric side quit. I replaced the thermostat and it has been another 4 years.
I think the average will be around 13-15 years. That is what the shop that sold me the "refurbished" one said. Refurbished in that shop was to replace the ammonia unit coils, and do nothing but clean the electronics, burner, wiring etc. I now know how easy it is to "refurbish" an old Dometic and will do my own if needed.