If the truck bed has internal tie-downs you may be able to run a cable from one rear tie-down around the backside of the camper to the other tie-down. This can be down low where visible and possibly even be run behind a piece of trim should there be any. Shouldn't take much to stop the rearward translation. 1/8" cable is probably too small, but not by a lot. Use a turn-buckle on one end to tension the cable. Armor the corners with steel though, cables make decent saws given the chance.
What is your bed mat's weight? Our is in the 50-60 lbs range. If yours isn't that heavy it may not be as much rubber as you need. Our first camper did the jitterbug on wash-board roads until I put that mat under it. It rides between the steel bed and the camper floor, nothing else is in there. If you have a plastic bed liner that for sure will give trouble, even with a rubber mat on top of it.