Only commercial trucks have to stop at weigh stations. Don't know about California.
There are some people here who are more knowledgable than I am about registering heavy trucks as RV's, but as a former CDL truck owner-operator, I advise you be prepared for some hassle.
Some on this board have done so without too much trouble; There's a guy on the Dodge/Freightliner Sprinter board who got rid of his Fuso FG because of the hassles w/ commercial vehicle authorities, though, too.
One work-around that might help in a few cases would be to register at or below 10,001 lbs GVW. If it's not being used commercially and is registered below 10,001 lbs, you should be exempt from pretty much any commercial vehicle authority anywhere.
At a federal level, a CDL is required above 26,000 lbs. New York is one of a few states that apply commercial vehicle laws above 18,000lbs or some other number.
Laws as to what weight you're allowed to register a medium-duty truck probably vary from state to state but FG cab and chassis is less than 6000lbs, so in theory it would be possible to operate an FG under 10,001 lbs.
There are a lot of private motorcoaches being operated well over 26,000 lbs which are not being driven by CDL-licensed drivers or being subjected to commercial vehicle authorities.
Kind of depends what you feel comfortable with and how much you can afford to be wrong....