Fresh tank overflow was introduced with the 4-way Anderson Valve which isn't on any 2011 Heartland trailer. You have a gravity fill for the fresh tank. Next to it, there's a 1/2" vent with screening on it. That's the vent that lets air out of the fresh tank as water is added.
The fresh tank has a drain - not for overflow - that lets you empty the tank. As the fresh tank is usually located above the axles for weight distribution, the drain is typically found in front of the front axle, on the off-door-side.