Some folks will stuff items into that space via the turnbuckle access hatches, but on most trucks, there's less room there than you think, once you've got the turnbuckles, etc. in there. (Also, depending on which camper/cabinet layout you have, those spaces aren't the most accessible.)
Attaching anything to the camper is tougher unless you're going to attach it after the camper is in the bed and never pull it back out - otherwise you wouldn't be able to slide past the wheel wells. But yes, I suppose one could make use of that space, as long as you didn't need to get into it very often to service the items, etc.
(Alternatively, replace the truck bed with something like the SherpTek bed, and you could drop the sides for easy access.)