Had this same question regarding my 2021 Hawk (with water heater). So I ran this experiment with the water heater out of the loop (I closed off both the upper and lower water heater drain cocks):
1. Truck placed on level ground.
2. Filled the empty 20 gallon tank, 1 gallon at a time, until tank was full, and monitored
the "Full-2/3-1/3-E" inside red-light meter after each gallon added.
3. Opened the small drain cock on the outside of the camper, measured 1 gallon outflow one gallon at a time and concurrently monitored the meter.
4. Continued this process until drain outflow completely stopped, recording the results at each gallon.
5. Put the camper on a steep grade, aft end pointing downwards, and measured the remaining outflow.
This experiment gave me two sets of readings (on filling, and then on emptying). I ran the experiment twice, and got four sets of readings.
Being a retired hydrologist (This is why I had time to run this experiment
), I expected exactly identical readings each time, but this was not the case which I thought was peculiar. Here are my findings (averaged):
[SIZE=10pt] Min [/SIZE][SIZE=10pt] Max[/SIZE]
[SIZE=10pt]Full Light: 16 – 20 gallons remaining[/SIZE]
[SIZE=10pt]2/3 Light: 11 – 15.5 gallons remaining[/SIZE]
[SIZE=10pt]1/3 Light: 4 – 10.5 gallons remaining[/SIZE]
[SIZE=10pt] E Light: 0 – 3.5 gallons remaining[/SIZE]
[SIZE=10pt] +6 gallons in WH[/SIZE]
Based on these results, I never fire up the water heater after the "E" light comes on, even though, hypothetically, there may be 3.5 gallons in the tank and 6 gallons in the water heater.
Hope this is helpful.