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.