What sort of a fridge is the 12V/120V one that's there now? Is it a compressor fridge or an absorption fridge?
If it's a compressor fridge, I would try leaving it as it is, as power consumption should be comparatively modest (though greater than a propane fridge, of course). A reasonable solar and battery installation would be capable of keeping it running.
If it's an absorption fridge, go with propane--and it may just be a matter of hooking it up if it's actually a three way fridge.
My hunch is that it's probably a compressor fridge. It's hard saying not knowing, though.