Typically, there is a self-resetting circuit breaker or fuse within a few feet of the trailer batteries. It should be on the (+) positive phase cable. It may be in a junction box or attached to the frame/tongue.
Are you trying to charge with the tow vehicle or have you connected to 120V? If the batteries were not disconnected from the usual parasitic loads of the trailer (like the radio, alarms, circuit boards), they may have been totally discharged while left in storage.
Having a hand-held meter to do some diagnosing is the tool to have when working with the 12V system. You could measure the battery voltage and the truck's voltage at the charge pin on the umbilical to verify that charge current is going into the batteries.