I have a MultiPlus 12 | 800 | 35. With Corono the bus was just standing there for days. After say 4 days I found that my starting battery was drained (also auxiliary battery). That happened at least 3 times. Then I saw that the MultiPlus was set to "charge only". After switching to "off" no more problems.
All the time the car was not connected to an external AC source.