Camper Battery doesnt charge while going down the road

The fridge could be switching to DC power - the vehicle charge is not enough to keep up with the power drain from the fridge is running on DC power.
That's the first thing that you should check.

Second, after the truck is warmed up and running for a few minutes, check the camper battery (with a VOM) to see if it actually is getting charged from the truck.

After that, you can do all that other stuff that was recommended.