If you have 1/2" clearance above the bed rails now, laying a sheet of 1/2" plywood under the camper would give you an inch of clearance and be fine.
The more important clearance to be concerned with, is the gap above the cab. If you have too little clearance there, the camper overhang hits the cab roof when driving on uneven terrain. The bed doesn't flex so much to need alot of clearance with the camper.
The overall bed/cab twist is much more and why the cab needs more clearance with the camper than the bed. This is of special concern those of use who drive off road or on primitive roads alot. On paved roads, it's not a big issue and less clearance is needed.