A converted 40' skoolie, eight people (and their stuff) and 7500 lbs of trailer? I'd want nothing less than a DT466, 8.3L Cummins or one of later ISB (6.7L)or ISC (8.9 L) Cummins. Pretty much any tranny that is in the bus will handle the load. I would aim for one of Allison's 1000, 2000, 2500 or 3000 series trannies. I would avoid Allison's AT545 (no lockup torque converter).
This rig will not be a screamer and will be slow up steep grades.