old water tank as holding tank
just one word of caution, make sure that the old water tank material is compatible with sewage.
fiberglass might be ok, but most metal tanks aren't.
but if it is metal, you might be able to yank it out, cut the top off, put a good few layers of really thick epoxy, or something to cover/conceal the metal, and then put the top back on (maybe a few layers of fiberglass tape).
i had gone through this thought process too.
but i figured that it would be easier to make a tank, stitch and glue style, than to refurb/convert the actual old tank.
using the drawing in the manual, you can do a little math and figure out all the lengths and angles on each piece of wood. then just a little(lot) bit of epoxy, and you're in business!
but i like your idea of using that space to house the holding tank.
Converting Water Tank to Holding Tank
Well sorry guys, I didn't follow thru with this thread, as to what we finally did. I have pictures of the conversion and installation, that I will post in the gallery. We did figure out that the tank is indeed monel. (I have forgotten now the "research time" involved with this). We figured at least three years of use, before having to replace with a plastic tank. We purchased a Wilcox-Crittedeon compact manual head. We then proceeded to purchase other plumbing parts. Some from hardware stores and others from Defender, and the local West Marine. After spending apporx. $350.00 we have a head with a holding tank of approx. 8.5 gals. Check out the pictures, then you can post your questions, and I will try to answer them the best that I can.
Fair Winds