I've made real progress.
http://www.zbasic.net/forum/viewtopic.p ... ight=#8697While I haven't yet done a test on either of the tanks in my '99V, I've determined that capacitive measurement is pretty easy and accurate, at least in a five-gallon bucket - if you are an electronic experimenter.
I suspect I'll ultimately be able to reduce the project to fit inside a Xintex 2" case, but I think I'd rather have the fresh water level displayed near the galley. Now that I think about it, though, there's no reason why it couldn't be displayed at two positions, galley and head.
Are there any other electronic builders here?