Priced this build out this week, and realistically, I'm probably looking at around $100k (roughly) once you factor in the truck.
I think that's very reasonable considering what poorly built motorhomes of a comparable size sell for, but also probably a bit beyond what I can spend this year. I'm not wondering if a mid-sized Skoolie might hit a lot of the wish list items are a lower budget.
Of course, doing a school bus reno comes with it's own set of challenges...
I've read that people
ordering a truck and waiting for it to be built are getting much better pricing than those buying a new (or used) truck off the lot. (You may already be aware of this.)
The ability to simply switch chassis cab trucks as needed is quite useful.
I know you want to build a truck camper rather than tow a camper trailer, but considering your financial realization,
perhaps you might (for the time being) consider a double axle utility trailer and a DIY camper?
Keep things simple. Water jugs instead of tanks, LED headlamps, propane stove, coolers with extra insulation, and lots of wipes and sponge baths.
Avoid really hot camping unless you have installed a small 120V AC and have access to 120V outlets. Avoid really cold camping unless everyone sleeps with fresh water to keep it from freezing.
Although lumber and plywood are much more expensive than a few years ago,
building and using a camper like this might get you using it quite quickly and relatively cheaply. It would also be a bit of an experiment to see what works for you.
When you do build and use your truck camper, you could also pull your trailer camper
if you want the extra room (or adult privacy).
Depending on the age of your kids, they might be interested in using the trailer camper at home (if it is safe enough for them to do so).
...............................
It was a number of years ago, and the plywood and Rustoleum paint may have changed, but,
you might find the following interesting:
A while back, I designed and built a small camper trailer to tow behind my Jeep. 45 degree angle at the back for departure angle. Same size rims and tires as on my Jeep. Same track-width. Torsion axle stubs. I welded up my frame with a receiver hitch front and rear on the trailer. Pintle ring inserted in to the front receiver on the trailer. Pintle hook in the rear receiver on my Jeep. Tongue long enough that combined with the pintle hook-ring setup, I could turn with the tongue more than 90 degrees from straight ahead towing, without Jeep body to trailer body contact. The trailer body was built out of 2x layed flat and 3/8" plywood, with 3/4" plywood floor. Silicone on every joint and sheetrock screws. No insulation or interior sheathing, so very easy to see how all the joints held up.
I painted the outside with Rustoleum white metal paint. I towed it across the US. It spent a lot of time in the Pacific Northwest rain (and some snow), but had no leaks when I sold it 5 years later.
I "glued and screwed" every bit of the perimeter, on every panel, to something solid. The "glue" did the sealing and the screws pulled the panels tight and clamped them until the "glue" cured. I was careful to avoid pushing all of the "glue" out of the joints.
I painted multiple coats of Rustoleum metal paint (white) and let the ACX plywood soak up all it could, especially the edges.
It was just ACX plywood from the store with the cheap orange buckets.
The camper trailer was easy and cheap to build and seal. 
en.wikipedia.org
www.westsystem.com
