You could buy a clip on amp meter, put it on the truck battery
lead, shut the truck down in the normal manner, unplug the camper from the truck.
Note the reading on the meter, it should be near zero, if it is reading something on the meter, start to remove fuses in the truck to see where the parasitic current drain is taking place..
If the truck comes up clean (nothing using energy from the batteries), plug in the camper and note the reading, do the same thing to isolate
items of energy consumption in the camper that are normally off, unless turned on..