The fridge on the moterhome we had when I was a kid was a Dometic
"two way" - 110 and propane only.  That's still a pretty common type
of fridge.  Clearly, such fridges are meant to be run on propane
while driving, and I assume the same is true for the three-ways.

I'm using mine on 12V currently, because my propane tank is
leaky (and thus permanently empty).  I figure "cold while driving"
is better than no cold at all.  Once the tank's replaced, I'll probably
go back to propane most of the time.

                                        -Steven