My wife and I have lived in more than one school bus. We are currently living in a 1998 International amtran 300 in the Chihuahuan Desert of New Mexico. I know a lot of people have removed the ceilings , torn out the existing insulation, and spray foamed the underside of the roof. While that may give you a little more r value it's not necessary. There are other options that don't require removing 3 million screws. If you feel that you are losing too much heat through the roof in the winter time, and or gaining too much heat through the roof in the summertime, you could insulate insulate using foam board on the ceiling, instead of in the ceiling.
You lose a whole lot more Through the Windows then you will through your already insulated roof. Here in the desert the Sun beats down mercilessly on the roof and some heat radiates on the inside. Much more radiates Through the Windows. To solve this we covered most of the windows on the inside and the walls width 2 in pink foam. We then covered the roof with solar panels, which shade the roof and provide Power. We found our 327 watt panels on Craigslist from a fellow in Albuquerque for $140 a piece. 10 of them fit nicely on the roof providing ample power wow reducing radiant heat from the Sun.
I know it's disappointing when you expect help from friends and family members and it's not forthcoming, however you have a vision, if you are patient , take your time, and learn the skills that you need to complete the job, then plod along until it is done, you can accomplish what you set out to do.
An inexpensive way to make your ceiling look nice after you have covered it with insulation board is to use contact paper.
Ruth and I are living in our bus while doing our conversion. We are often crowded by materials and tools. My health being what it is sometimes even small aspects of the project take many days, or don't seem to progress at all.
We don't let it get us down we just know that some things will get done on another day. Best of luck to you. I hope you don't lose heart.