We have been having problems with batteries going flat on a couple of our 1250RTs. Apparently BMW have a software issue that they know about, but no fix yet. Basically on some bikes if using the BMW charger through the aux socket, it is actually charging, but won't maintain and is running the batteries down. They are recommending fitting a tail direct to the battery. I do not know if this is happening with Optimate Canbus chargers as well, but my own bike (1250) is on an Optimate and a brand new 21 1250RT is as well and the batteries seem to be fine.
I don't know any further details, but they also mentioned their factory fitted trackers are also running the batteries flat.