OLD Sundance 5th - water tank drain

[KenfromNORcal]

My old 2008 Sundance 3012RE 5th freshwater tank has never been used as far as I can tell (I bought it used 10 years ago). Thought I'd clean up the tank and use it on a camping trip. What I thought is the drain - a 1/2" plastic tube with a brass valve hanging from the belly - broke off. I would have expected to see water pouring out but it's not. Any ideas about this issue? Maybe the drain hole in the tank is clogged? Is there another drain somewhere? Thanks!

 

[david-steph2018]

What that tube could be, just a guess, is an overflow and the previous owner installed a valve on the end of it.
On our rig, the water tank drain is a blue line. But being 17 years old, the line could be different. The end could be a screw on cap or a valve.