I had a 8' 8" flat bed on my '05 Chevy dually. I carried an 11' camper. I always wished my rear axle was back further, I think it would have made the ride better. I always thought if I did it again I'd get a cab and chassis truck with an 84" cab to axle and install an 11' flatbed. This would require finding a camper that is flat across the bottom instead of dropping down past the 8' mark. I know there are several available, I'm not sure about the ones you're looking at.
My bed was 8' wide. I always wished it were narrower. I would make the bed exactly as wide as the wheels, don't know what that dimension is on the trucks you're looking at.
I had actual boxes on the sides and under the bed. they were lockable. I kept stuff in there all the time even when not camping: tools, fluids, tow-strap, jumper cables, extension cords, chocks, etc. I find having tool boxes on a truck so much more handy than a "normal" pick-up bed.