There is alot of loan companies that will not accept payments from 2nd or 3rd parties unless the loan is being paid in full!  You might get away with it a few payment then the questions will start.  You will need to read the fine print on her contract as well, some have statements against assuming payments.

Why does she need 2 payment?  This sound kind of fishy to me.

Things that could go wrong:

She could change her mind and end up just putting the payment money in her pocket and the loan go in to default and you will be out a truck and the money. 

you wreck it and insurance not want to pay, because you carry the insurance and the truck is in another name.  This is the way the isurance company will get out of paying, also in the fine print.  No matter what the insurance man tells you, read it for yourself first! 

Why not just purchase the trailer for what she owes on it?  Have you seen the title, or bill of sale proving that she owns it or its in her name.  Is she going through a divorce and trying to get some fast cash?  2 payments most likely means that she's already 1 behind.  I wouldn't do it.  Just purchase the unit if its something that you want.

Here is a big problem is when you get done making the payments and the loan has been paid off the title is going to go the orginal lender and not you.  Than you have to get the title from them, and this could prove to be a giant headache cause your name won't be on the title and the bank won't let you put your name on the title while they have a loan against it.  So when the loan is paid off and the title is sent out they could take that title and sell the trailer right out from underneath you and you couldn't do a thing about it cause your name isn't on the title.  If i was the loan officer(and I am a loan officer at a bank) I would be wondering why you would be making payments on someone elses loan.  You are better off refinancing the loan through your own bank and gettin the title put in your name.