I am on the hunt for ideas for storing along the back window on the inside of the truck. We do not take the camper off the truck so this could be used often, as in my husband's lightweight golf bag or...
Those are great ideas for storage systems, but I think the OP is looking for something to allow her to store things up in the WINDOW area. She did mention "gun rack."
It is a rather novel idea if you ask me. The rear window in the cab is pretty much useless with the camper loaded. Blocking it would not be an issue.
Frankly, I think most people just work on their PHD (Pile it Higher and Deeper).
Does anybody actually use the back seats of their trucks to carry PEOPLE?