Your tank should be high enough to let your vent line be short, straight and relatively horizontal--which, if you go with 1" ID, should eliminate the need for a second vent line.
Loop your hose up and around before going down to keep water out of the vent.
That's a good idea for fresh water
and fuel tank
vents, but not for waste tanks
'cuz keeping the tank aerobic requires a straight vent line. If the thru-hull has to be in a location that'll put it under water when the boat's heeled, a clamshell cover will prevent taking on any water.....but, unlike fuel
and fresh water, it doesn't matter if a little sea water gets into a waste tank. Splashing won't catch enough sea water to notice.
However, you do want both the inlet fitting and the vent fitting on the tank to be next to the edge that's closest to the centerline of the boat, NOT next the edge that's closest to the hull
. This prevents tank contents from spilling out the vent or running back toward the toilet when the boat is heeled.