I am assuming you 73 is very similar to my 53 MY.
Starting Aft:
Fill tank port transom leads to Aft Watertank
Aft water tank has a large diameter Black Hose that leads through the Aft bilge to the lowest mid tank. The clamp to the mid tank at forward end of bilge.
In the master stateroom closet there are a few hatches. A view point to the "Gauges" centerline ish and a thin larger hatch that will show you the center tank and a few thin channels that run under the VIP and can make it up to the engine room.
From this long thin hatch you can see the black pipe that leads to the 3 water tank under the midline VIP bed. If you look at the VIP bed hatch you see the top of the water tanks and that pesky gauge that I can't get off
. Off the back (aft) of that tank you can see light to the closet hatch and see the connector hose.
You also have a vent connection to the Port side from the top of that tank.
I was worried we where not filling that tank, so I cut off (like the gauges, couldn't get the copper off). You can then blow out the vent with an air compressor to check for leaks.
A blocked vent may keep water from leaving that tank.
Do not blow air into the tank. I hear it can crack easily.
Shop vac the outside of the vent on the hull may also help.
Of interest (or concern). When I cut the vent I found that if you are filling the tank and that water will come out of that vent tube before the tank was filled. (perhaps a strange 1 off). I installed a check valve (Clear Tube between the copper so I can see water flow and didn't get it on other fills.
