Fresh water holding tank overflow HELP please

Since yours is a custom job its hard to know how it is set up. I will say that my Jayco fiver certainly has an overflow tube. As a matter of fact, that's how I know my tank is full. When you say you overflowed yours, how was it flowing? A tube, hose, or just emptying out of the bottom?

Edit: Biscuit's speed typing course paid off! Lol