Hopefully you are using a Volt Meter to test for voltage and also an Ohm Meter to check for continuity.
I would take my Volt Meter and check for the incoming 12 volts from the battery to the back of the converter. If you don't have 12 volts then I would put my meter on the ohm scale and check to see if you have continuity on the Positive and the Negative leads from the battery to the back of the converter. Make sure you have the Shore Power disconnected when doing this.
2004 Jay Feather 25E
2001 Ford Expedition 4X4 EB 5.4 3.55 rear
2004 Ford Expedition 4X4 EB 5.4 3.73 rear