I can't see your camper but most do have a 7 pin connector, it should have a 12 volt charge line if it's like most of them. I can't imagine someone building a camper without one.
When you install the 7 pin in the bed of the truck I would recommend connecting the 12 volt + line to a voltage controlled relay, also known as an Automatic Charge Relay (ACR). Blue Sea makes a nice one but their are others. This will monitor the voltage coming from the truck; when it is above the set point of about 13.2 volts it will connect the camper and truck batteries together so they will charge. When you turn the truck off the voltage will drop and the ACR will disconnect preventing you from running down the truck battery. If you get an ACR that senses voltage on both batteries and hook up a solar panel to the camper the ACR will sense the rise in camper battery voltage when the sun is shinning and connect the batteries to charge up the truck battery.
Just connect the solar panel to a charge controller and connect the charge controller to your camper battery.
