As noted above, operating on DC is only adequate to maintain the existing temperature in the fridge. It is not adequate as a substitute for AC or propane. I would also suggest putting an amp meter on the hot lead from the truck to the camper batteries. I think a good many folks will be surprised to find how low the incoming amps draw. Not really an issue if you have a couple of batteries in good shape, but might be an issue with a single battery of questionable ability.
Steve and Andra
2012 F350 6.2 gasser SRW LB
Fab Fours front and rear in case we run into a rhino
2019 Northstar Laredo SC