8 inches above the height of the box is more than enough room. In your post you never said which way it is out of level..?? Nose high or tail high..? You should be able to adjust your 5th wheel hitch in your truck and the pin box on the 5th wheel to bring it into a better level stance. Other things to consider, were your tires inflated to the right amount, do you have the correct tires on your truck..? Should you be adding a leaf or air bags to your truck..?
Like stated above pics would be great.
Glen & Robyn (A.K.A. Puff & Crickit) Full Timers..
2006 F350 6.0 with 3.73 gears, 4X4, Extra-Cab Long Box, Oil/Tranny/P.S. Coolers. Coolant filter. SCT Livewire.
2011 Cedar Creek 36RE, Auto levelers + too much to list..!