How many are using the HTT? The fresh water tank is full if the overflow line is pouring out from under the HTT. Make sure the tank drain is closed too; I have forgotten to close it once or twice, but have always noticed it before leaving the filling station. Just a note, the air vent in the gravity fill port is not connected to any thing and is plugged on the inside.
We have been out with two adults and two kids for 5 days on a single tank. No showers and being reasonably careful with water consumption. We also use the CG bathroom during the day and no showers in the HTT. I carry a 5 gallon jug to refill as needed; I connect a water bandit to the end potable tank, then a small hose filler onto that. This combo makes it easy to refill the tank as needed.
What probably uses more water than you think is doing dishes, and washing hands.