As you can see from these photos from my Flagstaff 8526 5th wheel, the 1/3 light will remain on until the water falls below the button sensor.
This can happen with 1/2 a tank of water if the camper is not level.
Additionally, the water pump is a suction type and will not suck water the second the top of the suction port is exposed to air. Again that point will be determined by several things including location of the port and levelness of the camper.
If you need every last drop, find out which end of your camper's fresh water tank has the suction line and tilt the camper to that side.
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