I have been on the road between Temecula, and Borego Springs. You are correct in that you are restricted to coaches of 40' or less on that roadway. It is definitely possible to make it with a 45' without any problem, and California does not post the restriction. I think it is unlikely you would be stopped or ticketed, particularly if you are an out of state vehicle. That being said, the reason for the restriction is that it is a narrow road with sharp turns. The state is concerned that you could not make the tightest curves, and remain in your lane. If you were to be involved in an accident because of straying into the other lane you would likely incur increased liability. Brian E. made that trip about 3-months ago, and he could possibly comment on the drive as well.
Steve & Rhonda Bennett
Dana Point, CA
92 Marathon XL40
2012 Jeep Rubicon Unlimited
Owner: California Coach Company, LLC