If you add a portable holding tank do you have a way to empty that tank? Or are you considering just extending the time between pump outs?
As said above do not leave the black tank open unless you want a huge pain in the a-- when (not if) the tanks plugs up. Whether you are using the factory tanks or an added on dump tank leave the factory dump valve closed until black is at least 2/3 full.
It's okay to leave the gray valve open the whole time. Though many people leave it closed as well and use the contents of the gray to wash all the black contents out of the dump hose.