Move the gas tank under the bed and have the filling line in the bed - now there's an idea I wished I never had.
I don't see what the problem is with having it under the bed as GM did for a couple of years (47/48) with the pickup and 8 years with the panel and suburban (47/55. One of the benefits from doing this allows you to move the seat back for better leg room and cab space. I see no issues with having the tank in the cab myself, but if I had a need to move it under the bed using the suburban tank I would have no hesitations.
It looks good with the body lines too.