It isn't really the camper directly behind the cab. I get splatters to the sides of the cab on the part that sticks out, and platters all the way across in a few inch band just below the overhang - where the 90 angle is. If I open the back window it is very different, camper/no camper. There is also more/different buffeting with the windows open, and more wind noise generally.
It isn't a surprise to me that different truck/camper combinations are different. There are many variables in shape, gap, height, overhang, shape of camper nose.