One thing about being on the road is no matter how peaceful a place may look when you go to sleep, it might be completely different when you wake up, possibly something going on you just don't want to mess with. Unless you're staying in an attended park every night, I prefer direct access to the driver seat, for those times when you'll just want to make a smooth quiet exit. That might not be possible if you're back there in a trailer somewhere.
If I was going to consider such a thing I'd consider a bus, maybe even an old school bus, they're built like a sherman tank, and would easily hold all your store supply, and leave as much room towards the front as you wanted for sleeping area. The floor in them is usually above the wheels leaving gobs of wasted, empty space on both sides along the chassis. Anyone able to fab boxes along the sides could increase storage space a Lot(sort of like the old Greyhound buses did for luggage).
Not sure what kind of mileage you could expect, but it could possibly be comparable to pulling a 30 ft box trailer.