Put something in writing specifying how much they are to pay each month, the pay-off deadline, your agreement to pay off back taxes if they pay of trailer by a specific date, and your course of action (what will the repurcussions be? That you won't pay back-taxes?) in the event of a default on payment (not on time, not paid up by the deadline, etc).
I suggest you do a consultation with an attorney and see if you can get him to yay/nay contract ideas/drafts (it is detailed, which will help... you always want to have specifics), or something similar that you draw up. Granted, it is only $550, so finding an attorney to do a free consult would be fantastic, but you will likely have to pay :-/ Anything in writing is better than heresay... just march over there with something just so you can say your agreement is in writing... I would write something like (keep in mind, I'm just a savvy layman and this DOES NOT constitute/substitute legal advice):
"I ________ (herein listed as seller) agree to sell my trailer to __________ (herein listed as buyer) in consideration of ____ amount of dollars, to be paid ___ times monthly, in the amount of x____. All payments must be paid by _____ date_____ of each month, and payment in full must be completed by ___date___. At the end of the payment period (__date__), buyer agrees to release title of trailer to seller. Buyer retains right to hold title until payment in full is received.
If all payments are on time and in full by the buyer, seller agrees to pay the back taxes owing on this trailer, in the amount of ____. In the event of late payment (even one day) or failure to pay in full by __date___, seller will not pay back taxes and buyer will be solely responsible for paying them in full, in the amount of _______."
Sounds official to me
Good luck! Seriously though, I'd march right over with a written agreement of some sort. Not sure your course of action if she refuses... hmmm... because ultimately, even though it was verbal, you made an agreement to sell your trailer to them and they've already started paying you for it...