Don't want to sound TOO biased but faced with the same problem we went with a dorm fridge and an inverter for less than half the price of a used unit- but there is no wrong way to do it
A good used fridge is hard to get
It may be easier to find a camper with a lot of water damage and part it out.
Good Luck, Mike
"wine is a constant proof that God loves us, and loves to see us happy" ben franklin