Had same problem, bought one inch think 1x6 decking, and screwed it in three places across the bottom of the TC ... from side to side, not length wise ...
Then bought a 3/8 think pre-formed rubber bed mat (I believe I bought it at Advance Auto, or some auto parts store, can't remember)that is shaped to fit the bed and curls around the wheel wells, laid it in the bed, dropped the camper on it, with the cross-bars and never have a problem. IT raised the camper one and a half inches, and cleared the side rails (was tight before, along the top rail), now fits perfect. If you ask, and want it, I will go over to the storage area tomorrow, and shoot a photo, which will explain it better than a thousand words.
It works really well, and have had ZERO sliding issues since TC new in 2008 ...
Monkey44
Cape Cod Ma & Central Fla
Chevy 2500HD 4x4 DC-SB
2008 Lance 845
Back-country camping fanatic