I believe you're actually talking about a BLACK water (toilet waste), commonly referred to as a "holding" tank. GRAY water is "galley,bath and shower
And as long as I have the floor for a moment...there are several flaws in your plan.
1. a 60 second toilet flush is at least twice as long as 99% of people would keep their finger on flush button or pump a manual toilet and would fill up a 40 gal. holding tank
very quickly if a toilet were wired to flush that long after every use, even after urine only.
2. The entire tank must be well above waterline to be capable of dumping via gravity. And because gravity is what causes liquids to run downhill whether you want them to or not, gravity doesn't require any help from a pump. The simplicity of a gravity dump tank is its appeal. 3. If your tank isn't high enough in the boat
to use gravity to dump it, an automatic timer on the macerator is not a great idea unless there will always be the same amount in the tank and the timer can be set to run only long enough to remove that amount. More in the tank, it won't be emptied...less in the tank will cause the macerator pump to run dry...and running dry "fries" the impeller in a macerator pump.
4. Dockside pumpout equipment
has strong pumps that PULL waste out of a tank. A pump in the pumpout line would just create a blockage that would make pumpout impossible.
So I think you're trying to waaay over-engineer a fairly simple system...and in doing so, creating problems instead of solving 'em.
p.s. Sorry but I can't resist adding: Most people believe "if it ain't broke, don't fix it." But to an engineer
, if it ain't broke it doesn't have enough features yet."