If you set the switch to store, the coach batteries will not charge.
The drain isn’t an issue when plugged in - the converter will support the 12v load and charge/maintain the batteries.
As HaRVey stated, and specific to the Mercedes chassis listed in your profile, nothing but the alternator will charge the chassis battery. That is a Mercedes restriction.
Not yet retired...
Florida (Michigan transplant)
2014 Hurricane 32A
2000 Infinity (previous)