A li'l gadget called the Uniseal is the way to go UNISEAL
(Scroll all the way through to learn how to use it and to get to the price
Drill your hole (at least 1" btw) insert the Uniseal according to directions..a piece of pvc pipe or a pvc elbow
...and you have a vent fitting that WILL NOT LEAK. Btw...vent line should be no longer than 5' (shorter is better), straight as an arrow, and rise no more than 45 degrees. Vent thru-hull should be an open bulkhead fitting, NOT a "vent" fitting.
If you want to make sure the tank contents stay in the tank and can't spill of out the vent or run back to the bowl when you're heeled, put the vent and inlet fittings away from the hull
, toward the centerline of the boat--preferably ON the top of the tank-- so that waste runs away from 'em on one tack, hoses are running uphill on the other.
Only hose I recommend is Trident 101 Trident Marine: Sanitation Hose
A double walled rubber hose that's been on the market nearly 20 years without a single
If you need more help in spec'ing all this out, I'll be glad to help you do it. Meanwhile, if you haven't already starting building this tank, you might want to check out Ronco Plastics (no relation to the VegoMatic Ronco) Ronco Plastics
They make TOP quality thick-walled (at least 3/8") rotomolded PE tanks
for a VERY reasonable price
and have more than 400 shapes and sizes, over 100 of which are non-rectangular. And they install fittings in the sizes and locations specified by the customer when they make the tank.