Under the cabin sole on the keel is a fine place for a water tank, but not for a holding tank, because the vent line has to be too long to keep the contents aerobic, which is necessary to prevent odor out the vent. For a detailed explanation, read the "Odor Control" section of the article at http://www.boatsafe.com/nauticalknowhow/phall.htm


There may also be another problem with keep that tank if it's _meta_l: urine is so corrosive that it will typically eat through a weld at a seam or a fitting within 2-5 years...and eventually will turn the bottom into a collander. I've yet to see a _meta_l holding tank last more than 10 years once it actually starts being used. Otoh, a good quality thick-walled (at least 3/8" for 40 gal or more) plastic tank will last for decades...as long as the boat lasts.

The best place for a waste holding tank on an old "step down" houseboat is under a bed, a bunk, or a settee that's within 6-8' of the toilet. Ronco has a 55 gallon tank that's only 11" high, which fits perfectly under most bunks. You may lose some drawer storage space, but that puts the tank close to the hull where the vent line (which, btw, should be at least 1" instead of the "standard" 5/8"...1.5" is even better) can be short, straight and horizontal, the head discharge hose going into the tank can be relatively short, and the pumpout line can be short AND mostly vertical.

Your boat is the same make/size as Joe's, with the head in the same location...and he's also doing some major refit. He and I spend the better part of an hour on the phone...I suggest you get with him to discuss how he's building out his boat and where we decided would be the best place for tanks.