RV Repair: Strange voltage seen

Question
My engine battery in my 2012 gas Newmar is being drained. I disconnected the neg cable. A day Later the dash radio was clicking and I heard the CD drive motor running. I checked the constant 12v supply and it showed 19.1 volts. I momentarily hit the MH battery "boost" switch and the voltage dropped to 13.1v where  where the coach batteries would expect to be. Where could this 19 volts be coming from? It remained constant for at least 15 minutes. This would be a likely place to start looking for the battery drain right?

Answer
This seems really strange did the hot wire you unhooked touch the frame. That could cause voltage to travel back to the system. The battery drain sounds like the isloator that splits the charging sysyem I would start there and the proceed to the fuse box and use amp ring and see if you can find the high amp draw