I just completed a bus conversion here in Ca. Like you, my original title said automobile, BUT under body type it said BU for bus and that does not exempt you from the smog requirements now being placed on diesel vehicles. To get that exemption, the body type needs to be changed to MH for motorized home. This change requires that the bus be permanently modified for living purposes as spelled out in CVC 362. I made the necessary mods and then hired a licensed vehicle verifier to verify the changes (avoided CHP and DMV inspection that way) and armed with that I went to DMV and had no problem with the body type change.
Another issue is that unless you have the MH (sometimes they use HC-- house car-- but both end up with the same definition), if you only have a class C drivers license you may be driving "out of class" unless your bus is registered as a MH. I'll bet that if you had an accident under those conditions your insurance would be void and you WOULD be cited for driving out of class. Take a look at the DMV recreational vehicle and trailer handbook https://www.dmv.ca.gov/portal/wcm/co...df?MOD=AJPERES
and study page 3 very carefully as it is a little tricky to follow.
Back to the smog stuff again. Is your bus a two stroke diesel? eg Detroit Diesel 8v92 0r DD 6v71. These engines are almost impossible to retrofit with the particulate trap etc that is now required on commercial diesels (including buses) and is the main reason these buses are being sold off. The smog retrofit costs between $10,000 and $20,000 and effectively precludes the commercial use of 2 stroke engines in Ca. Thanks CARB! So far, once these diesel vehicles are converted to motor homes, they do not require smog cert.
I hope this has been helpful. Let me know if I need to clarify any part of it and good luck. Jack
P.S. The verification is done on form 31:
P.S., P.S. When you hunt for insurance, ALWAYS call your bus a MOTOR HOME because that is what it is. If you say bus, the insurance company will throw you to the wolves.