Here is a link I found posted on irv2.com forum:
State Towing Laws for RVs
rv laws by state. looks like 45' is it for max length on a single unit. I know there are longer units out there, I have seen coach builders like Kingsley up to 50'. Anything over 45' you'd be over legal, but the odds of getting stopped would have to be exceedingly slim. The dot guys that inspect commercial trucks have no authority over RV's, and you'd need to find a really bored and/or ambitious regular cop to go "hmm, I wonder if that RV is 45 or 48'", and then take the time to pull you over and actually get out the tape measure. They both have plenty of ordinary law breakers to keep them busy with far less effort. There are a ton of guys on that forum with pusher/stacker combos in the 70-80' combined (65' is legal) range that claim towing all over the country like that for years with no issues. I've never been to CA, but they are reputed to be pretty tough on oversize rigs there. I'm assuming of course that the driver of a rig like that is smart enough to not get into a situation where you are stuck and blocking traffic in some tight place you should not be in the first place, where the cops get called to direct traffic around you or some such.