X2 for a DC clamp on ammeter and voltmeter. Check the battery voltage at high idle with the trailer attached. Check the amp draw on the trailer wire. Load test the truck and trailer batteries.
Maintaining the truck battery should be a non issue for the alternator. If the trailer draws to many amps the truck fuse to the trailer should blow, check to see what size is currently installed.
Did your "people testers" apply a significant load to the truck battery to confirm that it's being charged at the maximum? Unless you know for absolute certain that the truck battery and alternator are good...
