Because the vent comes into the tank below the tank's waterline you will of course lose a lot of your fresh water into the bilge, that is not how PSC did the original installation, the vents always go up, never down. The vent originally went up inside the transom into the cavity in between the cockpit and the stern.

If you install a deck fill then you will need to put a through-hull vent back there so you can see when the tank is full.

When you remove the water tank top cover you will see that the vent is a small through-hull fitting at the aft end of the tank, your best bet for a fill pipe is to install another of those fittings but with an elbow pointing up. This fitting should be 3/4" hose size and go up to the inner cockpit side where a male through-hull hose fitting (with cap and chain) can be mounted to screw directly to a garden hose for filling. You know tank is full when water comes out through the vent.

With the double wall construction of the tank it is realy only possible to use a standard 1 1/2" water fill right through the tank cover and that of course would cut into the quarter berth space by about 8" or more.

Hope this helps, Matt