I think a good cleaning would make the old holding tank suitable for fresh water for showering, doing dishes with soap, etc. I have never drunk the water from any tank whether in a camper or a boat; with routine applications of small amounts of bleach it would not be pleasant even if absolutely clean. We carry at least four gallons of store bought water for making coffee and drinking. I always get distilled water as it, then, doubles for refilling the batteries.
Having said the above, I left my 20 gallon holding tank in place (with a new boat I had the luxury of it being unused) when I installed my Air Head–which works great, BTW. By simply setting the Y-valve to "suck out" you can fill the tank with fresh water from a hose stuck into the pump-out fitting on the stern–just the opposite of what it was designed to do. Disconnect (by cutting) the tube to the overboard discharge through-the-hull fitting at the Y-valve and let it set near the rudder steering mechanism. Connect a new three foot length of tubing to the Y-valve's discharge port and lead it into the starboard deck lazarette near the trim tab pump. Insert into the end of the tube a male barb-to-female garden hose fitting and seal it with a cheap plastic ball valve-to-male garden hose fitting. When you run out of fresh water from the main tank, attach your 15' white potable hose to the garden hose fitting just mentioned and open the fresh water inlet (you can do this from inside the boat at the window above the stove-clever Ranger people!) and run the hose inside through the cabin, out the window and let it dangle into the fresh water inlet. The narrow side deck makes this a nightmare to try from outside the boat in any wave action. Now for the cool part: turn on the macerator (its a really fast pump) with the Y-valve to overboard discharge and pump water from the "holding" tank through the hose into the main fresh water tank. It's exactly 30 gallons in the holding tank so your water is doubled and no need to ration showers when you're out on the hook for a few days.
When you order your Air Head ask Geoffrey to install a female threaded port into the liquid tank (tell him "The left side of the tank when you are sitting on the toilet"). I had this done on my spare tank (always get two, the spare fits perfectly under the sink in the head), if the tank fills up at night you (or your wife for sure) will appreciate the spare. Buy a standard threaded male nipple-to-3/8" hose barb and screw it into the tank's port. Add a couple inches of hose to a male/female snap fitting (PM me for details). One fitting dangles off the tank and the other goes to a 20' long 3/8 plastic tube stored under the sink out of view, through a hole in the floor, out past the water heater, into the lazaretto, and back towards the trim tab pump and around and under the bait well towards the rudder steering mechanism. Remember that discharge hose you cut off the Y-valve and left dangle there? Well spray the last 10' of the 3/8" tubing with WD-40 and thread it through the discharge hose. That will neatly lead the tube to port and around the batteries and right up to the discharge through-the-hull fitting. Remove the hose from the fitting and insert into the tubing you pull from that end of the hose a barb-to-female garden hose fitting. Screw the fitting to the male end of the through-the-hull plastic discharge elbow.
Now, when your Air Head liquid tank fills up, remove it and set it on the toilet for elevation and snap together the tubing fittings to let it drain by gravity right out the old macerator discharge fitting that came with the boat. I put a little piece of that rubbery fabric for inside kitchen cabinets on the toilet top so the tank won't slide off and spill–Ugh!! It takes about 20 minutes to drain even with 3/8" tubing because the path is tortuous. But it sure beats carrying a sloshing pee tank along the dock up to the bathroom (at my Florida marina through two restaurants on the dock) yelling out "Bringing out the dead!" to all in my way. Now I store my spare tank under the V-berth as I don't need it.
You can get fancy and install a galley pump somewhere in the tubing, but do not use a pump with neoprene fittings–they dissolve in the presence of uric acid. I think the Whale pumps are OK. But, why bother... Remember, urine is sterile with no nutrient value so dumping it overboard is not harmful. It is gross, though, and smelly so if you are in the marina do it at night or do the walk to the bathroom after the restaurants close...
