When I was at the dealership I asked the same question of the technician working on my rig (I am a NEWBIE to the RV world).
He showed me how to fill the tank and what to expect. As all do -he put the hose in the tank nozzle and began to fill it up. Then he walks away. I am thinking OK it is a big tank and will take a while.
So I was not to concerned. However, after time went by - water began to spill out from under the rig.
I am like what the..........
I asked the technician what was going on. He told me to fill the fresh water tank let the hose run till it comes out the overflow. Then you know you are full. However, When I checked my gauge it showed full.
It never made it to the filler neck the hose was inserted into.
Question, was the RV level?If not, The level guage may read differently?
I hope this is helpful.