It's a bigger Mercedes thing, though my 608 never suffered from it. They have an isolator switch for the batteries, but when it's switched off neither the alarm or doors work. The funny thing is, the colder it gets the more it consumes. The Plaxton has it too, though I don't think everything is isolated. The tacho will take some, but that should milliamps.The engine Start Battery(s) should not be draining when vehicle off unless there is a leak somewhere or something left on, surely ?