Best thing would be to locate the tank(s) and that should give you an idea where to look for the deck fill. On my 1988 O'Day 322, I have 2 30 gallon water tanks located under the port and starboard settees. The deck fills on on either side of the top,deck, near the toe rail, amidships. My,last boat, a Hunter 280, had a 40 gallon tank under the V berth, and the deck fill was actually in the anchor locker.
Once you find the tanks, you should be able to hold a flashlight up against the side and see if there is water in there. Or turn on the water pump (usually controlled by a breaker on the DC panel) and turn on the faucet.
Cleaning them may be a challenge. Neither of my boats had a clean out in the tank. On my Hunter, I could remove the gauge device and get a shop vac nozzle in there to clean out any debris. Have not tried it on my O322. Bleach can be used to sanitize the tanks, but I don't care for the taste afterwords. Seems hard to get all of the bleach taste out. I have used hydrogen peroxide added to the tanks to kill any crud in the tank. If you each this site, or Google it, I am sure you can get some "recipes" for using bleach or peroxide.
If you have a hot water heater, that adds another thing to clean and check.
Good luck with it! I will be adding some hydrogen peroxide to my tanks now that we finally got to water turned on the marina. Hopefully letting the peroxide mixed with a few gallons of water sit in the tanks for week will kill any bugs in the tanks and keep,the water clean and sweet.
Greg
