I have the original 1959
LP-only fridge working in my Overlander. I had it pressure-checked by two different people, just to be sure: no leaks, and it cools like crazy. I did have to find a replacement door gasket and make some repairs to the interior door. And I have to keep the setting pretty low or things freeze. Also, I created a fresh-air vent in the access door, and an exhaust vent behind and slightly above the unit. There was NO intake before, and only exhaust was through a gap in wall insulation leading to small roof vent. Serious scorching in the wall led me to do something different!
My research on LP refrigerators indicates they can last a looooong time. There are even places that can make new ammonia-filled cooling units for them.
What will eventually cause me to have to get a new fridge will be failure of the Klixon valve, which is what opens & closes flow of gas to my burner. Apparently no one has figured out a suitable replacement. But your 1969 should have a more modern (and replaceable) thermocouple safety valve, I think.
Will try to find some photos of old set-up and new set-up to post.