You will likely have a panel that looks something like this:
Press the top center of the door and it should pop open.
Will have 12v DC fuses, and 120v AC breakers behind the door, the fan grill below is for the CONverter (120V AC to 12V DC/battery charger) Most trailers of this era will not have an INverter (12V DC to 120V AC) unless it was added by someone, and it could be installed just about anywhere.