If you are looking for electric only, a number of our folks here have used the compact fridges with great success.
They are just a tiny bit smaller than the standard RV fridge which means they'll fit right in the opening, but it'll take a bit of creativity and ingenuity to secure it in place and trim off the opening. Sizes range from 2 cubic foot and up. Prices generally $100 to $200.
Walmart lists 115 different ones on their website alone.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
2011 to present: '11 Cougar 326MKS
1999 to 2011: '99 Sunline T-2453
Amateur Radio kd2iat monitoring 146.52