I couldn't find this issue posted in the forum so I will post for those that may not know it is a problem with some 2008 units with 3 tanks (grey, galley and black)
I came across this as my 2008 8313SS unit which has 2 new 6V batteries. The batteries would go dead over a week or so sitting in the yard not plugged into shore power. I Found an article on another forum about this issue so I went and touched the grey water elbow heater and sure enough it was warm.
Basically the relay installed for the tank heaters is the wrong one. When the switch is off, the relay provides power to the grey/black tank heaters. When the switch is on, the grey/black tank heaters go off and the galley tank heater comes on.
The relay is under the shower on my model and the existing relay is a Tyco V23234-A1001-X036. As seen here
I replaced it with a BOSCH 1 ST/PC 0332019150-3PH Relay that allows two normally open circuits.
A little info on relays can be found here
for those of you like me who have no idea what 87, 30, 86 etc on your relay stand for....
Basically your relay needs to have two "87" or normally open circuits. The one it shipped with has on "87" and one "87a" (normally closed).
Simple $20 dollar fix that saves me hauling it to a dealer for a 2 week wait for repair.
Hope this might help others. Go feel you grey water elbow heater pad to see if its warm when the switch is in the off position.