First I would check the output of the convertor while plugged into city power. If nothing check the input side of the convertor to be sure it has power. If it has power but no output check any fuses that may be on the convertor.
Once you have output from the convertor check the trailer battery voltage. If you have both convertor output and battery voltage check the 12volt fuses or circuit barkers. I suspect the problem is before the fuse/barkers as it is highly unlikely that ALL would fail at the same time.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles