Yeah I wouldn't get to hung up on your battery voltage to chassis, it's probably just a ghost voltage with very little current, not even enough to light a 5w bulb I'd guess. Simplest way to eliminate and battery voltage tracking issues though is to simply lift out the battery and stand it to one side with it plugged into truck.

I'd check your multi plug connections on the bottom left hand side under the foot plate, they run up to lights and to access 1, they often get that blue corrosion going on inside the terminals, pop the little green plastic retainers from inside the multi plug and make sure they are all nice and clean.

How many hours has it done?