I spent several years living out of the back of my pickup with a shell and camping out under the stars a lot. There was also a couple years in a tipi and some of my nomadding was off the back of my motorcycle. In recent years Ive gone to SUVs, Suburbans then a 4 Runner currently. One of my criteria for buying a vehicle is being able to sleep in the back of it, even if I dont do it much these days.
Ive modified them all in some way to make them more useful. Ive built a dog platform in the back of the Suburbans and 4Runner, rather than the common sleeping platform and storage underneath, I dont like cutting down the headroom inside. The platform is roughly 3 feet front to back, and fits inside the wheel wells. It gives room for the dog to lay and see whats going on when driving, storage underneath and on the sides, and I can put my legs underneath it to sleep. I usually have a box of tools and assorted junk on the right side of the middle section, it doesnt interfere with sleeping or the dog moving about in normal use. The middle part has a 3" thick dog bed in it, and I keep blankets and a packing pad to level out the middle to rearmost floor. The folded down seats kill usable space, Ill take them out at some point and make a plywood floor with some storage underneath, and I can eliminate the step in the middle to rear floor that way as well.
I made plywood sides and front and rear panels for the platform, I button it up tight (bolts) when traveling with valuables inside, when around home, I leave the rear panel off and access it from the hatchback door, or leave the front panel off if the cargo rack is mounted in back. It takes about 5 minutes to remove the front panel to get to things to prep it for sleeping. I move stuff from under the platform to the side or the front seat to sleep.
Im not a fan of rooftop storage, beside catching air and potentially affecting fuel mileage, they arent easy to get in to, particularly if one has back problems. I use one of the hitch mounted cargo racks, I modified a folding adapter for the carrier and it raised the cargo rack about a foot, I had problems with the rack dragging on dips, curbs, and ditches in particular. After raising it, it hasnt snagged on anything. The folding adapter also allows a trailer to be towed with the cargo rack in place. I made a wood box on the rack, the size of the rack square, and as wide as the rack. It so far hasnt affected my fuel mileage any that I can tell after several cross country trips and keeps the inside less cluttered. In the summer it houses my small Honda generator to run the house type window AC unit to keep the dog cool when I have to stop and leave her in the truck. It will run about 6-7 hours on ECONO mode if you use a hard start capacitor to make starting the AC easier. Theres still about half the box left for cargo. In winter, its all cargo space. I put trailer lights and the licence plate on the back of the box so its more visible, it plugs into the trailer lights plug in the back of the truck.