paying before trailer is here

The bank loans the money to YOU- they have nothing to do with delivery dates of the object you're using the money for. All they care about is getting payments on the loan according to the schedule you agreed to.
Also:
Unless there's a clause in the loan papers that says the interest clock doesn't start ticking until you have the trailer, I'd think twice about asking for a delay in commencing payments. Could cost you a bundle in interest!

If there isn't a deliver-by clause in the order you placed with the dealer, there should have been...

Call the bank and see if they'll adjust. It's hard to believe they've already paid for a unit that hasn't been manufactured yet. They can only say yes or no... so I'd ask.

Yup, two different things. Sounds like the factory/dealer let you down on that one. Lender has nothing to do with it, and won't look kindly on you not paying. I'd be on the dealer if I were you.

I did sign but they told me the trailer would of been here by now. I called the factory they haven't started to build yet. My fault I guess

I bet your loan paperwork shows a start date for the loan payments....and I bet it's a next week date.

That all depends on the paperwork you signed for your loan. Every loan statement has a payment amount and start date.