After sitting at above 10 K feet elevation for well over 100 days, our black water reading became suspect from time to time. We restored accuracy by waiting till the black tank was nearly full, to empty it, then immediately refilling with rinse water. The factory installed rinse/flush system is probably the best solution.
When we left to travel to the next job in North Dakota, we added a bag of ICE to scrub the inside of the tank, which restored the accuracy by a better cleaning than a simple rinse.
Our volt meter has never been extremely accurate, but it has also never totaly wrong. It's not a precision instrument, but instead is intended to give us a general look at the battery condition. Our solar panels kept the battery voltage in the neighborhood of 13.2 daytime, and at night down to 12.2 .
The fresh water tank sending unit has never read 100%, though we know that it is full because of the overflow. On the bottom end it will start to suck air at about 8%.
I guess that for us, the bottom line is, that we understand that the readings are a best, a rough estimate of our consumption or replenishment.
That gives us a "heads up" to start thinking about a place to dump or to refill.