Getting a full fresh water tank is more "art" than "science" because each water tank has a unique shape and install (location and plumbing).
Because the orientation of the inlet and outlet of the tank varies, I will demonstrate with my install how it looks and what you can do to get the most water IN (keeping it there is another problem
In this photo you will see that when my camper is connected to my truck (how most folks will fill their fresh tank heading into their camp site), the tank floor is NOT level. In fact, it is quite "out of level" and the tank will not ever fill completely before the air vent is under water.
Worse yet, even while the vent is spurting water (full) the indicator lights will only show 2/3 full due to the location of the sensors (from bottom to top - 1/3 - 2/3 - full).
Since the outlet to the water pump is side mounted, not all the water is available to the water pump either. The pump "sucks" the water out of the tank. Once the water level drops to the very top of the outlet pipe, air will enter the system and water will stop flowing.
Depending on how level you are, will determine how much is usable (never 100%).
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW