I have a 12v (Edgestar) refrigerator I keep in the back of my van. I plug into a power outlet go to a Group 34 AGM (Sears Platinum) battery and then to the refrigerator. I unplug from the power outlet at night to prevent any drain on the vehicle battery.
If I wanted to charge a trailer battery during the day while I was roaming around, I would make a little battery holder to slip into the receiver hitch and fix a 7-way connector with just 12v and ground wires to clip to the battery posts.
Prior to all the electronics, I used to start the tow vehicle and swap out the batteries while the alternator kept the engine running.