I use engine driven A/C for driving.. the only reason to use an electric unit while driving is if you want it to also function while parked.. ie a camper A/C over the driver area run by an inverter and alternator .. which in turn can be used for A/C when you are parked on generator or shore power..
12 / 24 volt DC moile A/C is low BTU and wont begin to cool you on the road.. even in a rear engine bus.. you can reliably get 40-50k BTU out of a single compressorrun by the engine.. a sanden enhanced SD7 will give you upwards of 60-70k..
in my DEV bus I gutted the driver heater box and installed a heat / cool evaporator unit.. its about 20,000 BTU A/C and I think 25,000 BTU heat.. that runs my driver heat and A/C.. i left my original right-side defrost / door heater in.. although I completed rebuilt it.. this gives me extra heat for cold winter into the right side of the glass and the cold stepwell..
I ducted out dashboard vents and floor vents that i can open and close as desired for the heat / cool (left) side.. a separate heater unit was installed to operate the left side defrost.. I split the air intake of the defrost unit from outside air and off the evaporator of the heat / cool.. so in effect if I turn on the compressor in the driver unit i have dehumdified defrost like your normal automobile has.. an 8x8x8 area you may want a slightly larger heat / cool unit.. I have a 40-50k as my midship heat / cool cabinet unit.. ive got the lines and such running to it to get as much as i can out of it.. which it does real well.. my bus is all stock no tint / factory walls, seats, factory everything so it requires lots of A/C..
there is an in-dash evaporator thats made for Thomas HDX's
this one is a somewhat shallow depth heat/cool ducted where you can set for heat or defrost with a dial.. and has 30k BTU of A/C..
something like this could likely be mounted in dash next to or inplace of ther step well system..
theres all kinds of stuff.. I used an oopen-frame device with flex ducts coming off it and moubted it inside the old original heater cabinet which I had gutted out since it was broken and the coil was leaky..
with my dash air, the midship and now i inmstalled a rear system.. im able to keep the whole bus in the hot 102 degree dallas heat in the mid 70s inside on the road..