Here's what we had when towing a TT.
I had a campershell (cap) put on the truck with a pass through Boot between the back window of the cab and the front window of the campershell.
I attached a 4 foot wide one inch thick piece of plywood long enough to fit from side to side on the campershell lip/ bed rail. I placed 2 inch foam rubber on the plywood and covered it with indoor outdoor carpet. This provided a bed for the two dogs. They used to lay there and look out the through the pass through as we drove down the road.
The air from the dash A/C kept them cool and they could walk around on the bed. I kept a non-spill water dish on the bed in the corner.