Grounds! Grounds! Grounds!
And once checked, check again!
If you replaced a circuit board in your converter - you typically can only replace the board in an old linear converter - and those linear converters are a death sentence to your batteries.
Replace it with one of the newer multi-stage microprocessor controlled converters. They make ”drop-in” replacement converters for most of the old linear units out there.
I like Progressive Dynamics. Go to their website to cross reference to your replacement - then search Amazon for the best pice (at least that’s been my experience in upgrading our 2 coaches.
‘91 Ultrastar Champion‘02 Georgie Boy Landau 8.1l Workhorse
‘03 Jeep Wrangler TJ 2018 Jeep Wrangler JK toad