I need to replace my holding tanks. I seem to be unable to find the rather odd shapped ones like my original tanks and the available "stock" tanks end up being too small. So I guess we built tanks (it'll be good practice for the bus conversion. :P)
is one of the best how-to articles on how to make a holding tank (actually two types of holding tanks) that I've found. Look under "Resource & Refference" for "Good Old Boat Magazine" then click on "Archives". Click on "Systems" then scroll down to "Marine Sanitation Devices" and click to get the 10 pg article. (here's the link
If you want to know how many gallons a specific sized tank will hold then you need to either bookmark this site
or you can save it (as a web page) to your computer and the calculator still works without needing to be connected to the internet.