OK, this sound familiar . . last year I kept getting the same readings of discharge - I finally traced the problem to the 30amp blade fuse in the engine compartment - its the fuse from the alternator, mine looked ok but putting the multimeter across it resulted in no reading.
Also check the blade fuse holder - mine was partially burnt out where the fuse was arcing across the fuse holder - a new fuse holder & [good make - not chinese crap] fuse cured my problem . . hope it cures yours - let us know
Also check the blade fuse holder - mine was partially burnt out where the fuse was arcing across the fuse holder - a new fuse holder & [good make - not chinese crap] fuse cured my problem . . hope it cures yours - let us know