It is smells like rotten-eggs i would check the battery fluid levels... Your older trailer may only have a single mode converter/charger in it and it is well known here that batteries will boil out their fluids being on the 13.6VDC charger for long lengths of item...
Hopefully if your are low and haven't been in this condition you can just add distilled water and be ok...
When mine did this I ended with a shorted cell after I thought i caught mine quick enough and the battery failed...
A fully charged battery sitting with no load should read 12.6-7VDC across the terminals. With a shorted cell mine read 5.6VDC at the terminals (GAME OVER)
Roy Ken