black/gray tank monitors are notoriously inaccurate. most of us simply don't rely on them, some install aftermarket sensors.
you can use back flush or wand flush devices to try and clear the sensors but it doesn't always work.
after nearly 7 years of ownership, i have gotten to the point of looking down the toilet hole for the black tank and down the shower drain for the gray tank.
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package