fresh water filling

decided to fill my duetto 2005  fresh water tank from home before we went away in case I had trouble at the site , had no way of telling how full it was expecting when full water would come back at the cap entrance , but no , after a short water came out from the topand sides of tank itself thought I had a split etc in tank , called out service guy , apparently was coming from overflow pipe etc , seemed a lot , anybody any ideas on filling without wasting water please .
thanks
mike

Listen to the noise from inside the filler hose. (Assuming you have a Fiamma "hole in the wall" filler.)

You can hear the change in tone quite clearly as the water level begins to rise up the hose. Ours does pour back out of the filler cap if I don't stop in time, but that probably depends on how fast the hosepipe is running?

Don't worry about it. They all do it, and without an overflow hole in the top of the tank you wouldn't be able to fill it.