Hi Bob, welcome to the forum. I believe if you find the VIN, it will tell you the year it was made using a VIN decoder, which you can find doing an internet search.
Others have had your problem too. They've been told it is often stamped somewhere on the trailer tongue. I don't know where on the tongue that would be, so look everywhere. Mine's on a paper label inside the door, but that can easily go missing. Did the trailer have a license plate? If so, it was registered at some point in some state. Possibly you could find the VIN either on the registration certificate or, maybe the MVD in that state could help you, through the license plate number.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew