Our Alpines don't have a drain for water. Alpines don't have air brakes and don't have as much need for one. I installed a separate one to help drain the water, to a fixed plug that is on the bottom of the tank, and connected to the hose that goes to the valve near the air chuck that E. Mike talks about. The provided solution was to do as Mike says -- open the air valve and let it blow for a few seconds with the air up so it has pressure to blow it out.
I installed a separate valve with a pull cord like truckers use at the bottom of the tank, with a tee, and ran the pull cord to the side of the coach in front of the rear axle, to help drain the tank out of the bottom. This was part of my "low air suspension" issue thread on this forum.
I can also say that since I installed that drain valve in February in freezing temperatures in Texas, and have driven it 5,000 mile since, I have yet to get a drop of air out of the tank. Didn't get water out of the other tank drain solution, either. We just don't build a lot of water in that tank.
Former Owner, 2006 36MDDS
2010 Arctic Fox 22GK Travel Trailer
2007 Toyota Tundra