You've got some good advice here and some bad advice so let me sort through it for you. For info, I'm a retired Investigator with a local prosecutors office as well as a former police detective and I have worked these very cases in California and I have worked with DMV investigators on these cases. First of all, you state this is one of the larger dealers in Southern California. That is a good thing as there is very little chance you are in danger of losing any money on this matter. They have a very large bond on file with DMV as well as their own business insurance otherwise DMV would have already shut them down. The small dealers that set up in old gas stations are the ones that give DMV the most problems with these types of situations as they don't keep their bond active and have been known to run up hundreds of thousands of dollars in fraud like this and then take off to another Country, or file BK. You likely don't have that worry with a big dealership.
What the dealer is doing is like somebody else already pointed out. He's hoping to turn your motorhome around and sell it and then use that money to pay off the loan. Otherwise, he has to pay off your loan with his existing line of credit and then he has to pay interest on that until he sells your motorhome. This practice is not legal. It's possible he has his credit line maxed out and maybe can't even pay the loan off.
At this point you do not need a lawyer nor do you need to contact the District Attorney or Attorney General. You can either opt to tell him you are going to DMV if he doesn't resolve this right now, or at this point you can go straight to DMV. I would be extremely surprised if DMV could not resolve this issue for you in very short order. There are other things that they may also chose to do based on your complaint. They have the right to go in at any time and inspect his books to see who else he is doing this too. DMV also has two types of enforcement powers on this dealer. They have administrative powers as well as criminal powers. From the Administrative side they can fine him and/or suspend or revoke his license. From the criminal end DMV has investigators that are peace officers. They can go in and arrest the dealer if that is what needs to be done, but odds are it will never get to that.
In the extremely unlikely event that DMV cannot resolve this issue for you, your next stop is the Consumer Fraud Unit of the Orange County District Attorney. They can initiate either a civil action or a criminal action. Bottom line, at this point, you DO NOT need to pay any money to a lawyer to resolve this. You pay a lot of tax money and now's your chance to use up some of it to have the government agencies you pay taxes for resolve it for you.