Tent Trailer Battery Charging Battery In A Tent Trailer?

Charging battery in a tent trailer? - tent trailer battery

I've recently bought, a used tent trailer and just by an electrical connection for my truck, there are limits, electric lighting electric trailer brakes, fire prevention, home battery, etc. The house of the supply negative pole of the battery is constant if the truck or not. My question is: when I inserted the plug into the electrical system to the car and the battery must be recharged house, not for download from the battery of the truck when the engine is not running. The trailer is a power converter, I suspect that the cable, which, before it is connected to the battery. Thank you for your answers.