I live in California and have some experience with this, though I can't speak to issues with the threatened diesel ban.
Below is what I experienced and learned; having said that, the DMV is a bit like Oz, so your mileage may vary from these observations.
First, as soon as you can, start your DMV paperwork to convert the bus to a passenger vehicle. This is essential because you'll be hit with pretty exorbitant fees (based on weight) if the vehicle is not converted. As you probably know California requires continuous registration unless you explicitly register the vehicle an non-operable, and those fees keep ticking upwards with penalties. If you're thinking you'll avoid the whole registration thing while to take a few years to convert it in the back yard, think again.
Sidebar: my first trip to the DMV, they 'converted' the bus and charged normal automobile fees; my renewal was hung up because they screwed up the paperwork-namely, failed to require a vehicle inspection, which I was then required to do. I learned they screwed up because the guy who did my renewal spent a good twenty minutes talking with a superviser, who did not come to his help originally, just told him something that resulted in a fee of $4,200.00 showing up for me to pay. I just laughed, and sent him back to his supervisor. She was ultimately very helpful, and I appreciated her finally leaning in to sort things out.]
To pass the vehicle inspection that converts the commercial vehicle to an automobile you MUST remove most/all the seats so it is no longer useable as a bus (be used 'for hire'). Then you need to fill out a Verification of Vehicle form:
https://www.dmv.ca.gov/portal/file/v...le-reg-31-pdf/
and have the vehicle verified at the local DMV.
In addition, you may need to complete one or more Statements of Facts depending on your circumstances. In my case, we filled out a couple of them to capture details like I never drove the bus as a commercial vehicle during the year, or (filled out by the DMV superviser) that the guy who did the registration last year did it wrong:
https://www.dmv.ca.gov/portal/file/s...s-reg-256-pdf/
They'll have the form available at the counter if you need to state some facts.
Then you'll definitely need a Miscellaneous Certification:
https://www.dmv.ca.gov/portal/file/m...-reg-256a-pdf/
This form is the one stating you've converted the vehicle into a motorhome and you're certifying it will be used for human habitation (Section E). You can also use the form to indicate if you're tax exempt because you're from the Taipei Economic and Cultural Office (Section B) or if you're Native American and will only be driving the bus on the Res (Section C). They will ask your purchase price and cost of changes including labor. It is unclear to me whether those costs are used as a basis for calculating fees. I'll point out that they are asking for the costs associated with
converting it to be livable as an RV, so ONLY those costs associated with 'closets, cabinets, kitchen units or fixtures, and bath or toilet rooms' apply. I would say that fancy Victron system do not figure in to the conversion cost from their definition.
Regarding motorhome certification I will add that there was a lot of confusion in this space, and apparently the inspector who did the vehicle verification seemed not to have given any indication in the form that it had the things they ask for to call it a recreational vehicle (listed quite vaguely in the REG 256A form and mentioned above). Trust me when I say some DMV inspection folks, they need to smoke more weed because the one who did my inspection wanted me to stand six feet from her, speak only when spoken to, and follow her instructions exactly; and still she never stepped foot in the bus to check the conversion. Whoa. Chill, friend.
Like others have stated, DMV offices appear to vary widely in their experience with conversion paperwork and ability to interact with the public in a civil fashion towards the end of a long day, so be polite but
don't get angry or loud, state your needs, and if necessary ask for a manager if the person assisting you has never done one of these before.
I'll add one last bit of detail about my experience. When I was called up to the counter, the fellow behind the plexiglass was clearly new at the job, spoke very poor English, and was given the crappiest station right under the freakin' loudspeaker continuously screaming NOW SERVING A24 preventing us from hearing each other and literally triggering a rare migraine. Still, somehow we got through it and even bonded a little...