Sleep under a camper shell instead of truck camper?

At this time, it's almost totally for just sleeping when the need arises on the road. Might camp some, my gf wants to go some, has never been.

Does anyone know if I could open the rear window in the cab and cool the back under the shell at all? Will it blow up my gasoline engine, lol? (Actually, I just realized that the shell is not, literally, connected to the cab, I guess there is an air gap.)

The styrofoam mattress I have is soft and squishy, actually, it's really comfortable.