Jerry, I just went through a project this Fall reworking the holding tanks on my 38 footer. After several years experience with the previous system consisting of a 140 gallon fresh water tank and ~32 gallon holding tank, it seemed plenty obvious to me that the water tank was WAY too big and the holding tank was WAY too small. I could easily go a month on a tank full of water, but if you take a crew of 8 out for an afternoon on the water, you will just about fill a 32 gallon holding tank. (Your results may vary depending on how "intake" you allow and how often you dive in the water
) Keep in mind that even with a 3/4 full warning light (and this is ESSENTIAL) you will not want to press your luck to fill that last gallon of holding space. The downside is just too nasty to even discuss here on the forum. (voice of experience again) On the other hand, it is best to turn over your fresh water as often as practical. Even at the dock, I do not hook the water up directly to the city supply. This is both a safety issue and a way to insure that I continuously refresh my water tank..
I don't know if your toilet takes water from the holding tank or directly from the lake (or Ocean) but your big uses will be flushing, showering, and probably washing down the boat. Most people will bring bottled water with them to drink. We make our ice cubes from a bottle of distilled water we keep in the fridge. I don't know about you, but I don't do a lot of Ocean crossing where I would need a huge fresh water supply. For me, it is far easier to top off the fresh water than it is to go and pump out the holding tank.
So last Fall, I rerouted the waste lines to my 140 gallon water tank. (fiberglass in the keel) and put a new 37 gallon polyethylene tank in the space between the engines for water. I think this will fill our needs much better this coming summer.
I still have the old 12x20x29 waste poly tank if that's a size that fits in you application. You are welcome to have it. Obviously a waste use only.
Hope this helps.