The truck camper is already configured to have the battery charging from the truck alternator and shore power. Usually it is one or the other, driving or plugged in at a campsite or at home.
With solar the panels connect to a charge controller which is connected to the battery. It is providing power when the truck engine is off and when not connected to 120v AC shore power.
Amsolar is an excellent source for components and kits and technical information. Simplest to provide roughly 100 Watts solar for every 100 Ah or battery you have. The charge controller is the limiting factor in terms of how much current it can handle at different voltage levels. Start with the controller selection first and then the panels. I found it easy to download the manuals for all the charge controllers I was considering to get their tech specs and limitations.
The battery capacity is the single most important factor and I would not buy a new camper that was not designed to have two batteries straight from the factory.
