If you don't include a continuous duty solenoid it the charge line between the battery & pin 4 of the 7 pin connector, you can unplug the connector each time you stop for more than a half hour or so. The problem is if you forget, particularly if the refrigerator is running on 12V, a dead tow vehicle is likely. Far better to add the solenoid...
In my case I went with a #10 charge wire & a #12 brake controller wire. At best, I get 8 amps charging current on the charge line.
As Chandalen stated, it is also important to add a fuse. I'd go with a minimum of a #10 wire, however a short in a #10 wire between the battery & ground would draw enough current to melt the wire. While some individuals have run heavier wire to produce less voltage drop between the tow vehicle & the trailer battery, it is unlikely to provide much more than 5 - 10 amps. If you are lucky, it will be enough to keep the refrigerator running on 12V while you drive.
