I agree; you are good to go. DC is DC and which ever is the higher voltage is gonna come out on top. If the truck is able to generate a higher voltage than the camper's charger that's plugged into the house AC, then the truck is going to provide the current at that time. Turn off the truck and it will revert back to the current path provided via the camper's charger.
Same with a solar panel on the roof; you can parallel in the solar panel(s), via a solar regulator, to your batteries and there's no problem if you are plugged into the house power or your truck is running.
