DC current for mini fridge
Not so easy

The first thing you must learn is that you can't "dip" a vehicle battery very far, least of all your starting battery.  With lead-acid batteries, you can't use more than about 25% of its charge on a daily basis or you will degrade the battery and have short life.  

Lithium batteries are much better (and more expensive) and you can use most of their range daily, but even really good Apple quality ones are only good for about 1000 full cycles. 

Peltier fridges - too inefficient

A few "mini-fridges" operate on the Peltier principle using solid-state. This seems cool until you realize how ghastly inefficient they are. They do indeed operate internally on 12 voltish DC (with an AC adapter) but are rather weak and can only keep things cool, not chill them.  Far worse, their poor efficiency will necessitate a much larger battery.

House fridges can't handle vibration or shock

The #1 problem with residential grade refrigerators is they are not built for the vibration and shocks of being in a vehicle. Motion will tear them apart.  That's why they're out. 

Just informationally...

These machines use the normal freon-compressor cycle, which is efficient.  However they are AC-only and have a big AC draw when they start.  That requires a big inverter to drive them.  What's more, they don't run very often, but you don't know when, so you must run the big inverter 24x7.  A large inverter driving no load has a high "standby" energy cost, which saps your battery. 

"Other than that, Mrs. Lincoln"... people building alternative-power homes love the common residential-grade fridge.  They used to make very special fridges for solar off-grid use, with 6" thick walls - but it turns out there's no need, since recent Energy Star refrigerators are so good.  This presumes you already have a nice large inverter already in place for other reasons... and the battery bank to drive it. 

Ammonia (propane) refrigerators

Some refrigerators work in a triple mode:  120V, 12V or propane.  These work on a less efficient ammonia cycle - great on propane, weak on 12V.  It also introduces the risk of a propane leak into your RV, which can kill  you explosively. 

Freon refrigerators made for RVs

This is the best of all worlds.  The machine is sturdy enough to bounce around.  It uses the efficient Freon cycle.  It is either 12V or multi-voltage -- either using a DC motor internally, or a "brushless DC" (read: an AC motor driven by its own onboard inverter  that only runs when the motor is in use, so no standby losses). 

The best fridges actually open from the top - after all if you've ever been hit by a curtain of cold air when you open a vertical fridge, you know cold air wants to go down.  Fridges/freezers with top lids don't lose much cold when you open them.  

How do I know so much?  I follow people on the Internet who put refrigerators in vehicles.  You can too; look at 


British narrowboating (also called "on the cut", cut being a civil engineering term)
Boaters generally, but beware, boats don't hit potholes
#VanLife - the US "people who turn vans and delivery trucks into stealth RVs
Off-grid* folks using solar/wind power to homestead, they use 12V fridges, most of which are vehicle-rated




* In the "I don't have a mains electrical feed" sense of the term, heh heh