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Have your mom sign a statement something like, "In consideration for the release of lien on the OLDMOBILEHOMEDESCRIPTION, Borrower, hereby consents to registration of a lien on the NEWMOBILEHOME with the DMV, to secure the prommissory note between Lender and Borrower, made DATEOFORIGINALNOTE. As further consideration for this agreement, Borrower agrees to pay all of the net proceeds from the sale of the OLDMOBILEHOMEDESCRIPTION to Lender as partial payment on the balance of the promissory note."
Then you register your lien on the new mobile home with the DMV, and you're done.
I don't know if understand what you're trying to do.
If you want the buyer to assume your existing loan and lien, then just have the buyer sign a new note, consent to possessory lien, and then release the lien against your mom and stamp her note paid in full.
Now you're financing an arm's length third party. Don't charge more than 7%, or you're violating the usury laws.
Have your mom sign a new security agreement and note for the old coach, register the lien with the DMV, mark the old note paid in full, and then when the buyer appears, that person will have to either pay off the loan or take the coach subject to your lien, which you will have to transfer to the buyer when you register title in the buyer's name.