I once had a similar situation with a truck I bought. My advise: tell everyone you talk to as much of the full story as they want to hear. Don't change anything, don't embellish and don't lie. The truth has a ring to it that people will often recognize.
People often forge documents related to vehicle ownership in order to avoid sales tax or other costs/complications. The DMV knows this. If you tell them the truth they may want you to pay the previous owner's registration
or sales tax but probably not. Especially since the trailer was not previously instate.
I have found that if you can go to a smaller office and at a time when things are not too busy you will get a bit more consideration. If you go to a big office at 3:00 on a Friday you will get the legal line and nothing more.
Be nice when you talk to people. If you show them your problem and ask them what to do about it, most people will be glad to help. If you go in there like you are "it" you are more likely to be shown that you are not it.
I hope you got a good deal on your new-to-you RV because you are entitled to some compensation for fixing this mess.