Here is a link to one I did a few years back.
It might help you with a few things.
Hint: Campgrounds don't really like Skoolies but a reasonable paint job and decent conversion will get you into most.
A school bus yellow or psychedelic paint job will earn you a pass at most.
As for motor, diesel is way to go and moreso if you want to tow a truck.
I have 345IH gas motor with allison automatic and 60MPH is where it likes to run and gets 5MPG with AC running off genset.
Drives like really big suburban.
It is totally self contained but also has provisions to hook up at camp ground.BruinGilda the AeroSwine