I've got a box truck, a P30 in the parlance of the parts countermen. SBC and overdrive tranny - 8mpg - up hill, down hill, tail wind, doesn't matter. Mine is aluminum (strongly recommend this over steel) and made by Grumman as local delivery vehicles. Mine was onwed by a Mac Tools salesman and I bought it from him when he retired. Parts are plentiful and mechanicals are ordinary and dead nuts simple.
Readily available but being snapped up as food truck conversions in recent years (I've even had bits of paper left under the wiper asking if it's for sale). The point being, while they were once cheap and plentiful they can get pricey if you're not careful. That said, deals can be had.
They come in various lengths. I can close the doors on a 16' 2x12 if I let it bump up against the dog box. Mine shares a wheelbase with a dually extended cab Chevy I once parked next too at Wallyworld, as well as short school buses. Wouldn't want one any longer, or shorter. Be careful about widths. The ones UPS and FedEx presently drive are a good foot wider. Doesn't seem like much but this can make all the difference in fitting into a parking spot. Anyway, I've had it more than 15 years so these are the thoughts of a relatively long time owner.
I also have a hotshot. This is a Freightliner over the road truck with Caterpillar engine air suspension, etc. and equipped with sleeper but with a 26' box instead of being a bob tail with trailering provisions. Basically, they extend the frame and install a box like local deliver vehicles use. Mine has an aux axle, plus a hydraulic tail gate lift. Anyway, readily available.
The former is my do everything vehicle because I'm not keen on a pickup truck (or truck payments - I'm really adverse to monthly payments). With a kitty litter toilet and an air mattress, it will do in a pinch at a model airplane events for overnight accommodations, but that's what the later (the Freightliner) is for.
Anyway, I'm the wrong guy to ask about overlanding, or long term camping (like real RVs). Mine are all about taking to model airplane events as support vehicles (and doing little more than basic camping inside as opposed to being fully set up with all the amenities).
Note; FL allows registry of the latter as an RV as long as it has electricity, toilet, and cooking capabilities, e.g. this eliminates the requirements of a CDL. A diesel generator, a composting toilet, and a microwave do the trick plus a queen size inflatable mattress and we're set without having all that stuff underfoot all the time. I'm serious about just having where to work on model airplanes and this results in a large workbench and air conditioned comfort versus a pop up tent or needing to own a pick up just to be able to drag a camper to the weekend events (did I mention a non-fondness for P/U trucks?). I don't actually hate the things (pick ups) and have owned a couple but as a daily driver I would far rather use another vehicle (I usually drive my antique Rolls Royce or ride the Harley for errands). Note, I use the lift gate on the back of the big truck to take my Harley Sportster as aux wheels and it serves as a porch otherwise.
Meanwhile, I have been poking around on this forum trying to grok the why for converted school buses. I tripped over the site while searching for something else and realized you folks are solving the same kinds of problems as folks like me on the sister site to this one (same owner, I believe). The other site is
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Cheers!