My tank set up has the grey and black tank drains joining at a "Y" just before the connection point for the sewer hose. Each tank has a shut off valve so in order to gain more grey tank capacity when Boondocking I have done the following:
I installed an additional knife vale at the hose connection point which, when closed, will allow me to open the two tank valves to allow the grey tank to equalize volume into the black tank as the grey tank always fills up more quickly than the black tank. I can them shut off the valves if I feel it necessary.
The additional knife valve has the proper connector lugs so the hose can still be hooked up to drain. Does anyone see any downside to this arrangement?