Your Stereo runs off your starter battery the same way any car does, so that is what is draining your battery, it usually doesn't take too long either.?
My Euro is 1993 Westfalia. When I stop and camp,both the starting battery and auxiliary become drained if I use my cabin lights and stereo. I am wondering why my starter battery also gets drained. I was sure my starter battery should be protected in this situation??