What do you see being the issue with CA DMV?
My understanding is that if I buy a cutaway, CA DMV considers this an incomplete vehicle which cannot be immediately registered. You must have some sort of "certified" upfitter do the conversion and provide you with the appropriate documentation to certify it is safe and roadworthy.
If I buy an off the shelf box truck I can register immediately and then have it UJOR'd and do my DIY interior without any further lengthy dealings with the DMV.
Once I have completed my RV conversion, you go back to DMV and they change from commercial to recreation registration.
I could be wrong, but kinda think this is how it works.