you want to minimize the camper from siding.
http://www.amazon.com/Dee-Zee-DZ86928-Heavyweight-Bed/dp/B000VU91SGLook here for all kinds of related camper stuff :
Sometimes (depending on the model truck or camper) you might have to use something in the bed to raise the camper a little taller in order to give clearance between the truck cab roof and the cabover of the camper. see "camper loading,hauling,tie-downs"
'camper centering guides 1' (spott)
if you need to do that.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/25966285.cfm