As noted above check with ETRAILER.COM Great folks to work with...
I personally would think the adding the 7-way connections on the inside of the truck bed near the tailgate would be best solution but ETRAILER.com does have a 7-way Extension Cable setup to do what you are saying... ETRAILER has the one that will mount inside the truck bed and then plug into in the standard 7-way connection on the bumper of the truck. You do not need to tap into any of the truck wiring doing it this way.
Looks like this setup...

I too was thinking the TRUCK CAMPER uses a different setup connection to the truck. Shown above it what folks use to hookup to their 5th wheels from their truck bed. I was thinking the truck camper has a special looking cable to connect to the truck. Never dealt with that but do check out more what you need...
Roy Ken