One good option would be a pop up hardside such as the FWC models, they only stick about a foot above the cab when down and expand upward giving you plenty of space.
I actually have a cab over camper right now.
My plan is to go away from it.
I spent a lot of time looking at four wheel campers, but my issue is that I almost always need a LOT of cargo space. And I need the bed.
Here are the basic requirements:
All the stuff an overlander/RVer would want: Chairs, Hot water shower, Awning, inverter, full set of tools(like 300lbs of tools), Lights, Kitchen, BBQ, Stove, Fridge, RV deep cycle batteries, Air compressor, Solar, tow straps, shovels, high lift jack, etc, etc, etc. I already have all that stuff, and about 1/2 is already integrated into the truck.
Rig is a 2007 Dodge Cummins Mega Cab dually.
I built an 8 foot "utility" bed with 8' long x 15" x 18" storage boxes on the sides where the doors fold down to counter tops with Bamboo hard wood flooring for the counter surface. Then hung ProTech aluminum tool boxes under that.
The design is such that i am still able to load a cab over camper onto the bed. (It is still a full 5' wide at the tail gate between the boxes), the whole bed is 98" wide.
All that stuff mentioned above goes in the tool boxes.
I plan to also build a roof rack for the RTT, and solar, lights and awning.
With all that said, this leaves the entire bed available for cargo.
Unlike quite a few overlanders, I almost ALWAYS have another agenda on my trips. Either dirt bikes, Mountain bikes, or chasing desert racers.(most of the time).
This leaves the bed available for all that other stuff that varies from trip to trip, and the runs to home depot.
Edit:
Build thread is here:
http://sierrarockcrawlers.com/forum/viewtopic.php?f=2&t=22