My wife and I lived aboard our trawler for three years before the RV. We understand water usage. 50 gallons in three days is not normal. Our shower heads have on/off buttons for Navy showers to minimize water usage. On the boat, we could easily go for a month on our combined 200 gallon black/gray water tank before pumping out.
I don't know what the cycling on and off noise is, but I'm seriously leaning towards a major build up of crud in the tank as the problem. The only way for water to enter the tank is the drains, and since that isn't it, the tank capacity must be greatly diminished. I loaded Draino into it, with more to follow today. I'll dump the tank the next day and see what happens.
Thank you all for your thoughtful suggestions. This is, indeed, an odd one.
