Everytime I hear of these sympthoms I always think to check the installation of the battery terminals cables. It is so easy to get them reversed.
When they get reversed they will blow an in-line fuse between the battery and DC Power Distribution Panel. This will also blow two fuses in the Power Distribution Panel called "REVERSE POLARITY" fuses which are usually off to them self on the panel.
Items that are powered directly from the battery terminals will continue to work - Only those items that come off the Power Distribution Panel will not work with just the battery providing 12VDC.
Simple to check - Just fine the word "NEG" or the stamped symbol "-" on the battery case and physically hand trace the connected battery terminal cable and make sure this is the one that goes to the trailer FRAME GROUND which will be very close to the where the batteries are installed.
Once you get the battery connected properly then find all the blown fuses and replace them.
just my thoughts...
Roy Ken
