Sometimes a battery going bad will cause the converter/charger unit to do this... This happens alot when the battery fluids are being boiled out on some of the older trailers that don't have smart mode chargers.
The battery is showing shorted internal cores to the converter and it loads it down big time - then perhaps burns off the short somewhat and works normal again etc...
Using one of those clamp-on 400A Sears DC CURRENT METERs would show this clamped around the battery charge cable.
The ceiling lights all run off of 12VDC powered by the on-board converter/charger unit when on shore power.
Roy Ken