2006 Model 2619 Draining the fresh water tank

I just purchased my used TrailManor and have it in the back yard set up so I can become familiar with it before heading out for our first adventure next weekend. I decided to sanitize the 20 gallon fresh water tank and plumbing system using chlorine as suggested in the TM owners manual.

After filling the tank and letting it sit for the requisite time I went to drain the tank by opening the valve at the end of the tube extending down beneath the tank under the camper. Alas nothing came out! It would only empty when I started the water pump. This surprised me as I thought that gravity would do the job. Can anyone enlighten me on how the emptying of the fresh water tank is supposed to work?

Thanks!