All these are good posts, but if the beep goes away when you disconnect shore power, it's the converter in the trailer (that reduces 110V AC to 12V DC. This thing CAN overload (I think maybe it just gets too hot) and will alarm with a beep sound. It happened to me once. After disconnecting the shore power for a while, the problem disappeared and has not returned.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew