While not terribly helpful to solve your specific issue, all I can really suggest is: "ignore the gauges". As a general rule, I haven't even looked at the readings in the past year and a half of fulltiming. I know our main black tank, I can go about 2+ weeks. I know that 5 loads of laundry is the max for the back tank, and 6 will overflow it. I have no idea how many showers I can take, but we are usually in a campground so that isn't a big deal. Plus, if I'm boondocking, letting a little shower water out on the ground won't be a big deal.
2018 Grand Design 399TH (44') • 2109 Ford F-350 Dually
With 2016 Harley Heritage • 2013 Sportster with a sidecar!