Sorry for the very late reply...
We did it in California. But you have to catch the right DMV VIN verifier at the right time to complete REG form 31 to change the Body Type from DS (diesel semi) to motorhome (MH). It can be difficult to impossible in California.
By definition, motorhomes must contain permanently-installed independent life support systems [for human habitation] which meet the criteria of the American National Standard Institute (ANSI) and provide at least four of the following facilities: 1. cooking, 2. refrigeration or ice box, 3. self-contained toilet, 4. heating and/or air conditioning, 5. a portable water supply system including a faucet and sink, 6. a separate 120 volt electrical power supply and/or liquid petroleum (LP) gas supply. Human habitation is defined as living space which includes, but is not limited to: closets, cabinets, kitchen units or fixtures, and bath or toilet rooms. Many standard over-the-road HDT sleepers MAY come close with the addition of a porta potty. We have no sink, only bottled water and a porta potty. HOWEVER, in our case, the DMV VIN verifier never even got inside to look; only took our word! It saved us $2,000 per year in registration fees.
Make sure when you fill out REG form 256, Statement of Facts, Section G, to write exactly "This vehicle was permanently altered and equipped for human habitation meeting the definition of a motorhome according to CVC ยง 362." You'll need Reg 256A filled out, too. And bring your commercial plates and pink slip to DMV when you attempt this.
However, for INSURANCE, (in the spring of 2020) Progressive simply wanted us to disable commercial use by removing the commercial 5th wheel hitch and providing pictures of the truck, long before we got it reclassified Body Type Motorhome (MH) with DMV (in the spring of 2021).
Later (the winter of 2021) we had a box added to carry our Jeep in a "garage" (see pix) and some insurance companies refused to insure it because to them it could be used as a commercial box truck. However, since there is a permanently attached ramp (that does not retract) off the back, it is impossible to back the truck up to a loading dock and drive a forklift into the box to load pallets. Anyway, I skipped those insurance companies since at that time (2021) their premiums were more expensive than Progressive. HOWEVER, be forewarned, many insurance companies only allow for VERY limited situations where they will cover the truck BOBTAIL; like going to the repair shop or getting fuel. They only want to cover the whole combined rig including the towed trailer.
And finally, should you want to DOMCILE in South Dakota, we unhappily discovered insuring RV's in SD can be more expensive than in California!! Apparently, because of frequent hail damage claims in SD, and perhaps because of the shear numbers of RVs registered making claims of any type in SD (my personal guess).