Good Morning Slaytop,
Our tanks are not connected. I have seen the full light come on the panel and still be able to go for several days before the tank is actually full (as in the shower quits draining). I haven't tried it with the kitchen tank (my #2 gray tank) to see just how far we could go after it shows full. I won't recommend this but, with the sewer cap on, open both gray water valves and they should equalize. You'll have a spill when you take the cap off unless you have a gate valve at the sewer dump outlet. You can get them at Campers World.
John