I have a Northstar 8.5 Arrow mounted on my 2015 Chevy 2500, 4X4, long bed, single rear axle. A friend told me that when the truck motor is running that it is supplying power to the camper's on boar...
It is supplying power to the camper BATTERY itself, not the charger!
That is, if you installed a 40A fuse in the "Stud #1" position in the fuse panel under the hood on the driver's side. From the factory, there is a dummy fuse in that position.
Your truck outputs power for the 12V DC power system. Your camper's converter/charger operates on 120V AC household power.
Clearly the truck cannot power the camper's charger, but it can charge the camper battery directly when properly configured.