need to pick your brains

[Flatswing]

I would recommend against combining. Peggy Hall (the “headmistress”) would probably tell you to install 3 vents -1 separate for diesel tank with bug screen & 2 (preferably 1” diameter mushroom vents on both sides of hull, no bug screens) for your holding tank in order to facilitate maximum aerobic status of your holding tank (reduces odor). The mushrooms without screens, maximize airflow & allow you to back flush occasionally to prevent clogs , mud dauber wasp accretions etc.