Well, I got a hose extension that would bend down, and was able to fill it to what looks like 1/2 but of course still reads 1/3. But good enough, I don't need it full.
After 15 campers I don't believe those gauges. We've learned to estimate and haven't run out of water since camping the first time with our granddaughter.
I do press the buttons every once in a while for jollies.
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Those who know everything use pens. Intelligent people use pencils.
update: filling from a container worked better- I think I need a longer hose extension. Gauge seems okay as half full showed half full on the gauge. Thanks, Tommy, for the how-to-fill info and assistance!