But did you sign over the title and hand it to him?
That's important.
Go to the DMV ASAP, tell them your title is lost and you need a duplicate. You'll find out quick enough whose name is on the title. If it's you, buy a duplicate and go ahead and sell it and pay off your taxes. You take your chances on having trouble if you do that but if he squawks after it's gone you can probably buy off any problems before they come your way.
If it's his name on the title, treat the mobile home as abandoned and get the advice of an attorney as to how to get rid of it.
Meantime, I'm not sure I would trust the tax collector's statement about your land being at risk. He may just be trying to scare you into paying. Find out for sure by consulting a property tax expert.

