Maybe for fun try putting a charger on the batteries to bring them into the 13s range and see if that fixes it.Trouble with my Webasto. She's throwing fault code #3 which is an electrical issue -- too low or too high voltage. I've changed nothing in the van and all other systems work fine. Monitoring my Victron I see that upon startup the Webasto draws between 6 and 10 amps and watch my five D31A Yellowtop OPTIMA batteries providing 375 amp-hours start dropping from infinite time remaining to one day in about two minutes. I run this test with no other draw on the fully charged system besides some led indicator lights on rocker switches.
The books says: F 03 Undervoltage or overvoltage 84 The voltage was less than 10.5 V or 21 V for longer than 20 seconds 04 The voltage was more than 16 V or 32 V for longer than 6 seconds. My voltage never leaves the 12's