My deck is 9' long and you can get 8' of top boxes to clear the camper (my under deck boxes are 3'). I went with two 4' boxes on each side and they do not pass through to each other or to the camper. You can choose 8' boxes or go to 3+3+2 size boxes based on your needs. My popup canopy just fits in the 4' box without the rolling wheel bag it came with. The pictures with the camper off the truck were prior to my storage boxes. I bolted them to the deck and they stay on the truck when the camper comes off. I did not want add weight to the camper or its structure by attaching them to the camper since those four boxes empty weigh 450 lbs.
My tie downs are outboard of the storage boxes and must be removed to access storage but are easy to remove since they are quick release turnbuckles. Typical turn-to-tighten fasteners would be inconvenient for this application. You would have to frame over these boxes if you wanted to attach the camper above the access doors and there is insufficient room between the camper and boxes for tie downs.
The Eagle Cap and Host inside floors start above the boxes, so everything bellow is storage, tanks or generator which is where you you want to keep your heaviest items. The Class 5 trucks have much heavier springs and stabilizer bars than the Class 3 and show little signs of sway or porpoising even in stock configuration. You will sit over 13' high using a flat bed deck since the camper will sit at wheel well height of the typical pickup (about 3.5' off the ground).
Fully loaded and hitched, I have about 13,000 lbs on my rear axle and only use the upper StableLoad pads on the overload springs to keep everything at level. Without the StableLoads, I am slightly lower in the rear only when the 8000 lb trailer is connected.