You're correct that reducing the contact area increases the lb/sq-in pressure.
A few weeks ago I used my rubber bed mat as an exact "pattern" to cut a piece of 3/4" plywood to cover the front 4ft of the bed, placing a 1.5" spacer under the front (so my truck doors will clear the camper bed area), and 3/4" spacer under the rear, and a 1X6 across the front, vertically across the topside (to keep the camper from striking my P/U tail lights).
I fitted short 2X4s to the 3/4" plywood to center the camper, and screwed short 2X4s perpendicular to the longitudinal 2X4s, to reinforce the side longitudinal boards, and forming a storage box outside the camper's outside access door on the left side to restrain goods stored there.
The return 2600 mile trip "home," from where I installed the device, was uneventful, and the camper stayed perfectly centered and didn't budge fore-and-aft either, despite the elevated forward section.
In reality, my 2WD 1-ton dually sits tail-high, especially with Stable-Loads and air bags to keep the suspension from bottoming, so even with the 1.5" differential front-to-rear slope caused by the tapered spacer, the camper is just a tad higher in the back than the front.