Take a close look at the water input connections in the service compartment. It's pretty easy to connect the water hose to the tank flush connection instead of the fresh water intake connection. Some folks ( not me )have done that and filled the black tank till it flowed up and on to the roof.
It's possible to do that on my new coach too. I think I'll paint the flush connector a distinctive color, say brown??
Brenda & AL
2017 Thor Venetian, T42 "Groot"
2020 Chevy Sonic, 2019 KTM 790 Duke