I'd be really surprised that there would be only a single 40 gallon tank serving both the rear kitchen and the lavatory. Highly unlikely that they could get the plumbing to flow to it. My guess is that there is another valve, probably somewhere near the kitchen to dump the Gray #2 tank. Trying to run a valve cable from the UDC to the rear would also be a nightmare, and putting the valve near the UDC would have the piping full of liquid from the tank to the valve, which I don't think is permissible.
If all else fails, call Heartland tomorrow with your VIN and ask them how that rig is plumbed and valved.