Go back to the California DMV with all your information, including their new title. Print out a copy of this:
RVIA Multi-stage Manufactured Motorhomes. With all your information in hand and a copy of the RVIA information in hand, ask to speak to the DMV office supervisor and ask her/him if your title can be amended to show the correct final stage manufacturer and final date of manufacture. If that does not work, well, you are going to have to make a trip to the state capital main DMV office. Contact RVIA Government Affairs (703-620-6003) with your problem and they might intercede for you! Good luck.