I have a 2500 2003 Bigfoot...thought I would share my thoughts with you.
I have tried just about everything...Styrofoam 1/2"(lasts about 7 trips) 1/2" rubber mat(far too heavy), 1/4" rubber mat (self destroys and camper moves), plywood (camper moves)carpet under padding (self destroyed after 2 trips)...and a few more that I can't remember.
Bigfoots have a rough bottom because of the 2 large screw attached plastic panels to fix plumbing and workmanship problems. This creates a challenge to find a decent reliable pad that will not be damaged.
I realized finally that a single large pad spreads the load of the camper over infinite contact points and therefore does not make a solid point contact with the truck box.
The solution was to use black rubber holed 12"x12" anti fatigue tiles from Home Depot. Placed strategically on the truck bed they provide the required pressure points. 3 fore and aft and 2 sets of 2 amidships worked for me.
The camper will never move! In fact I drove about 5 miles once, before realizing I had forgot to put my tie downs on and there was no movement whatsoever...other than what appeared on my truck seat afterwards.
Hope this helps.