Your shore power powers up the converter/charger unit which produces 13.6VDC to operate your 12V items in the trailer including keeping the battery charged.
If none of the 12VDC items are working then something is disconnected between the converter and the battery and the 12VDC power distribution panel.
The problem could be a tripped circuit breaker feeding the converter/charger unit, blown fuses, between the output of the converter/charger and the 12VDC Distribution panel, and/or a run down battery or blown fuses or bad connections between the battery and the 12VDC distribution panel.
A multimeter will be very useful to troubleshoot the problem here.
Here is a typical 30AMP Wiring configuration that might give you a clue to where to look for the disconnect.
Roy Ken