Our much older, 2003, Winnebago has 3 drains. The large one you can see comes right out the bottom of the fresh water tank and down thru bottom of compartment. The 2nd, smaller, comes out the top of the tank and then dumps through the bottom of the compartment, that's the overflow. The 3rd, smaller like the overflow line, is near the water pump and is the low point drain to drain the interior lines. Tank drain and low point drain both have shutoffs. You sure your low point drain was closed? Was the water pump on? If the pump is on it will empty the tank through the low point drain.
Hank & Lynda
2003 Winnebago Adventurer 35U, Workhorse W22