I think the valve in my previous post photo is it and you just need to turn it. It's probably like the one in this photo and has 3 positions. The first would be normal allowing water to go from tank to water pump. The middle would be off and that would be your winterizing isolating the F/W tank and the third is the elusive tank drain.
2004 Winnebago Minnie 22e(went to a nice family Oct 3, 2015)
2016 Forester 2701DS (Aug 31, 2015)
2015 Camping 3 trips 6 nights
2016 Camping 9 trips 28 nights