As has been said, whenever the refrigerator is ON, you should always have the refrigerator vent fan turned ON when the TM is closed, regardless of the mode operation. The fan draws next to no power, so it has no effect on electrical stuff.
If you set the refrigerator to operate on DC while you travel, the refrig will take most of its operating power from the TM battery, and only a little from the tow vehicle. As a result, you will probably arrive at your destination with cold food and a dead or nearly dead TM battery. The way to prevent this is to turn off your TM battery as you travel - however, this disables the TM breakaway switch, which is a very bad idea. There is a separate thread going on right now about how to prevent this. Basically, you add a separate wire directly from the TM battery to the breakaway switch, so when you turn off the battery, the breakaway is still connected.
Take a look at post #16 here.
https://www.trailmanorowners.com/for...ad.php?t=20092
As has also been pointed out, the TM refrig is a pretty good passive cooler. If you get it nice and cold during the night, then turn it off as you leave your campsite, the food will still be cold when you arrive.
Bill