I have a 2018 GMC 3500 and a Bigfoot 25 c 94. I had to raise the camper 1" to avoid hitting the radius at the bottom of the tailgate opening. When loaded I have about 5/8" per side. In order to avoid hitting the side of the truck either loading or from shifting. I made front and rear guide blocks. The front ones are attached to full width, doubled up 2x4's ripped down to prevent camper from sliding forward ( I do not trust the two rubber bummers as the point load could damage the camper or truck bed). In the rear, the wedge guide blocks mount on a full width 1" board. The wedge blocks are 5" tall tapered for 2 1/2" to "guide" the camper into the exact centered position. Makes loading painless and no shifting during traveling.
