I don't live in the area but I talked with a guy who lives in Pomona had his Roadmaster base plates installed by Rose Auto Clinic in Montclair/Ontario on Holt Blvd. He thought they did a good job and the price was not outrageous like many of the RV dealerships charge. They are an authorized Roadmaster dealer.
For contact information if you'd like to at least talk with them, go to:
Rose Auto Clinic