Rodger,

That voltage drop is excessive. Something is puilling power from the batteries and it is enough of a draw of current that you should find it.

With the exception of the 3 circuit breakers in the rear electric panel that are for DDEC you should have no loads on the chassis batteries with the key off. At one time there was a discussion from Ben C. that DDEC on occasion would draw power from the chassis batteries with the key off. To rule that out the next time you put the bus away trip CBs #19,20, and 21 (verify they are for your DDEC) along with turning off the 12 and 24V master switches and see if you continue to have the voltage drop. If nothing has changed then at least you can rule out DDEC from doing it.

Be aware that if you disconnect power to DDEC via those breakers you will lose some data, but if you are not having problems that will not matter.

If you are confident you have no phantom loads it is within the realm of possibility you have a battery beginning to fail and you might consider disconnecting the cables and checking each battery individually. As new as they are that is a remote possibility, but doing the check rules out all but additional as yet unidentified phantom loads. It is easy to ignore them and just add a charger, but the reality is if you have something drawing current you need the peace of mind knowing it has circuit protection in the form of a fuse or circuit breaker and will not become a potential fire hazard.

FWIW I had a load on my original coach that took months to find and it turned out to be the heater in the Racor.