On my V3605 the tanks are,
Kitchen sink, front bathroom sink and shower all go to the front gray.
Only the back bathroom sink goes to the second gray tank.
Now, here is the kicker, the panel indicator on mine , says "Kitchen" for the back bathroom sink gray tank.
In small letters on the panel it says "tanks may be different depending on model".
When we purchased ours, at the walk through the dealer rep. told us the back bathroom sink and toilet shared the sam tank and the kitchen had it's own tank?. I know for a fact this is not true in my case. After tracking down the shotty warranty work the dealer did and fixing the leaking valve my self that the dealer didn't fix is how I know.
You could possible have a valve leaking into the drain as to why you do not see level on that tank.