My setup may be different than yours but, when I fill the fresh water tank, I always open the water pump compartment (curbside), where the tank is located, and watch the actual water level in the tank while filling it. On my tank, the overflow tube is a clear tube that comes out the top of the tank and goes through the bottom of the water pump compartment along with the low-point drains.
Gary Shapiro, 2011 Georgetown 29' Class A
Dash, Molly & Lily - now at the Bridge and forever in my heart
Shadow - 4-year-old Greyhound
Hannah - 1 1/2-year-old GSD rescue