The REVERSE POLARITY FUSE is nothing more than a BATTERY DISCONNECT FUSE. This tells me you have a parasitic drain perhaps the CO DETECTOR that is causing your drain.
This REVERSE POLARITY FUSE is in-line with your RED BATTERY TERMINAL CABLE and the BOTTOM of the #4 FUSE. If you can figure out which fuse in one of those bundled fuses on the back side of the ELX30 Converter you will probably find one POSITIVE lead going directly to the CO DETECTOR which is normal.
ALot of folks will install a 12VDC BATTERY DISCONNECT switch right off the POSITIVE Battery Terminal and open this up when the battery is in storage mode.
I have around the same 1/2A to 1AMP drain on my battery associated with parasitic drains. It will drain my battery down to 50% charge (12.0VDC) in about 3-4 days if left connected up. I just open up my battery disconnect switch when I am sitting at home and not on shore power.
My trailer however sets on shore power all the time when at home where I am connected to 120VAC 20AMP service coming from my garage. I do this to get my batteries charged up all the time using my smart mode on-board converter which is a PD9200 series which replaced my ELX-30 Single mode ELIXIR Converter right after I got it in 2008.
Roy Ken
