So your camper has an electrical system, it just isn't connected to the truck to charge the battery? there's different ways to do it, but I installed one of these, a battery isolator, that will allow it to charge from your alternator while the vehicle is running, and it will automatically switch between charging the car battery and charging the camper battery, and it will work like a one way valve, running down the camper battery won't deplete your car battery. https://www.amazon.com/gp/product/B00WTAFR84/
You just need to run a thick battery cable between your car battery and the camper battery, with this in the middle.
next steps would be adding solar panels, charge controller, and ability to charge from 120v shore power, but those are a lot more work.
Sounds like you have a fuse/circuit breaker open, a switch open, or a bad connection between your battery and the positive bus, or… a ground issue. It’s time to get the multimeter out.