I'd agree with the assumption that a commercial vehicle (ex: BlueBird) will indeed be built on a substantial chassis w/ sturdy coachwork.
You didn't mention the type of fuel used, either of which offers benefits and drawbacks.
Generally speaking the nature of stop and go driving that School buses are subjected to they are probably geared on the steeper final drive side. Meaning at expressway speeds the engine is revving near the upper limits of operation. I'd "guess" 7-10 mpg
But, I wouldn't think any rolling living quarters gets "good" fuel mileage.
We're all subject to compromises. Opting for stout makes good sense.
Best of luck in your quest.
J & J, DRV Suites ES-38RSSA #9679
GM Denali, 3500HD-Max, 4x CC, 8' DRW,
EZGo-TXT, Clubcar Precedent