Whenever the DC demand (load) exceeds the converter output you will get the alarm.....DC voltage at/below 12.5V.
The converter should be putting out at least 13.2 V DC with full load demand. DC output of 13.5---13.6V no load.
The converter will only charge your trailer battery when connected to 120V AC power source.......has nothing to do with truck 7 pin cord.
Truck alternator maintains charge on trailer battery when 7 pin cord is connected.
You need to:
Have battery load tested....take to an auto parts store
Check that ground cable from battery to frame is clean/tight connection
Check cable connections on converter...clean/tight
Check converter DC output when hooked up to 120V AC power source
Elixir Manual