There are two sides to every discussion. We use the city water hookup. Some water in some areas has a high sulphur content. You generally don't figure that out until the trip after you have it in your system and you start running some hot water. Then you get the rotten egg smell.
It doesn't take all that long to open the drain on the water heater and drain out the sulphur water. But if you have an 80 gallon freshwater tank filled with it, you may be flooding your site or the neighbors site to get rid of the bad water.
I do my best to fill my fresh tank at home with my known good well water. Not always possible but I do it when I can.
Black and grey tanks, leave them closed until you need to dump. Dump black first, then grey.