On our 44DS we have the same setup as the OP except we have one holding tank. My wife and I cruise for about 12 days with one shower per day each, washing dishes with fresh water, and fresh water heads, before I start thinking about stopping somewhere for water. We could use less with conservation. I believe the big user you described with 4 people aboard is the heads, seconded by dish washing. I can wash dishes for 2 with about 1 gal of water. The key is don't run rinse water extravagantly. Flushing the heads is another learned art. I flush until the water is clear in the bowl. Don't flush to be sure the waste has reached the holding tank as someone will follow on and flush again. Untrained guests are the killers on water. BTW, the advantage to fresh water toilets is no odor. Nothing. Also, the pumps are protected from salt water corrosion. We are in our 10th year with the Jabascos with no issues.