Every coach A/C unit I have ever seen had an engine driven compressor to make the A/C system work.
IMHO, if you have one in your bus it would be best to scrap it all out rather than try to make it work. The cost of maintaining coach A/C is cost prohibitive due in large part to the size of the system.
Most automotive A/C systems use less than 3 lbs. of Freon. Most coach systems use more than 10 lbs.--it take a lot to fill up the lines from the compressor to the evaporators to the condenser and back again. Some coach systems use in excess of 30 lbs. of Freon.
Because the lines are so long and there are so many joints in the system it is virtually impossible to seal the system up 100% over any period of time. Every time you go down the road you shake, rattle, and roll the system. All that shaking, rattling, and rolling contribute to joints loosening up and leaking.
The best solution would be to get some roof top A/C units and install an auxiliary alternator to power up a house battery bank/invertor to run the A/C while going down the road.
Whatever you do, make sure you size the unit appropriately. A too large unit running on low will cool things much better than a "just right sized" unit that has to run on high when things get hot outside.
I just spent a long weekend in our 1981 Avion 34V travel trailer. With outside ambient temps of over 105* our 13,500 BTU roof top struggled to maintain inside temps that were 20* cooler than outside. It takes a lot of BTU cooling to cool something that is more than 30' long inside.