There should be an emergency breakaway cable that is connected to a switch box on the tongue of the trailer. If that breakaway cable is pulled out of the switch box (or is missing completely), the brakes get emergency power drawn from the battery and will lock up. That will explain both battery being drained and the brakes locked up.
You can check your 7-pin connector using a voltmeter. With no brakes applied by the controller, and with the breakaway cable pin inserted in the switch box, you should have 0 volts between the brake pin and the ground pin on the 7-pin connector. If you have 12-volts between the two on the trailer side when it is disconnected (assuming a charged battery), then your emergency breakaway switch is activated.