On our 2001 F350, I made a 4' x 8' frame out of 2x4's (laying flat) and then attached a sheet of 1/2" plywood to that and then screwed the whole thing to the bottom of the camper. That made the spacer about 2" thick. Sounds like you need more than that. Maybe you could have two layers of 2x4's or one layer standing edgewise.
On my rig, the roof was closest to the cabover right in the middle of the roof. There was a lot more clearance at the side edge of the roof. This worked out well since most of the movement was on the side edges from the camper/bed rocking side to side.
Good luck,
bradw
