DC operation of fridge

The charge wire, even it usually is #10 size goes several loops before reaching camper battery, so the wire resistance plays the role.
We do have tens of topics how members who want better charging - run heavier wires either using Lance plug, or HD designated plugs.
Also beware that 12V fridge elements usually have much lower output, than 120V or propane outputs, so even when you solve the charging issue, the fridge will not perform the best with this option.

Check the voltage at the camper battery when the truck is off, then start the truck and check. If the voltage goes up you are charging. If not then you are not. (Don't plug in 110 for this test).

On my truck it needed a fuse so that the circuit became live. $2 and I had a pack of them.

Never run the fridge on 12V for any length of time. I would use 110 to cool it off at home, switch to propane when I pulled out of the driveway and off propane when I got back in the driveway from the trip. Most campgrounds I stayed at did not have power. Yes off when refueling as well.

My Truck is a 2015 Chev duramax Diesel 2500.
I have a 7 pin cord I believe.
I will check my fuse box on the truck for sure.
Will adding a solar panel really help with the on board battery charge?
We just returned for a first trip with the camper and put on over 7,000 kms. Included Saskatchewan,Alberta,British Columbia,Yukon and Alaska. We loved the camper and am excited to go on another trip.
Thanks to all for you help

Your truck may be charging the camper but the discharge rate from the fridge will be higher than the alternator charging so it may seem as though your truck is not charging when it is.

Try running the fridge on gas next trip and see if you get a charge. The charge through the 7 pin harness is a small trickle charge so don't expect a short drive to result in a big difference.

YES it is OK yo use propane while traveling...........one caution isto turn fridge OFF when fueling

As for camper battery NOT charging from truck...
Does your camper use a 4, 6 or 7 pin umbilical cord to plug into truck

Have to have a 'charge line' from truck to camper

Also check truck fuse box and see if a fuse/relay has to be installed for truck sytsem to charge camper battery

What truck do you have?
Some need to have fuse installed, some need to have wire terminated to get power to camper or trailer connector.
Yes, most people run on propane for refer while on the road. Remmeber it still takes 12 volts DC for many refers control.