Most important thing to know about the gauges in the F/W, grey and black tanks is that they are notoriously INACCURATE....to the point of near uselessness. We picked up our brand-spankin' new TT and the gauge on the black tank read Full. Some folks have changed theirs to a brand that is supposedly better, but no info here on the brand or exactly how satisfied they are now. If you go into the members listing and look for Chuck_S, he made the change, believe he has pictures of exactly how he did it and all the info you'd need. He most often is on the hybrid side of RVNet. Others may have the same info.
Usually the F/W tank is pretty much correct. The grey is likely close, but the black tank is anybody's guess. The slightest piece of debris of any kind can make the sensors go crazy. Even a good flushing of the black tank can still leave just a speck of TP or other and it'll show an incorrect reading.
After some use, you'll be able to know about what level you're at with the grey and black. I found, nearly the hard way, that when the kitchen sink starts draining very slowly that it's likely backing up into the shower. Our system is to be stingy with water going into the grey tank and very generous with the water for the black. 3 - 4 days is about our limit on grey and that's with 2 of us.