The low point drain near the fresh water fill should be for the tank itself. The valve is usually plain white. There should be two additional lot point drains for the lines in another location. They are usually color coded blue for cold and red for hot.
You said the tank was full when you left the dealer and you just finished filling it at home. It sounds like the valve is stuck in the drain position and it drained the first time on the way home and again now. As others have said the handle in line with the pipe is open (tank draining), the handle 90 degrees to the pipe is closed (picture of my closed valve attached).
If it won't turn try using a liberal dose of WD40. ONLY turn it by hand, it is just plastic. If that doesn't free it I would take it back to the dealer. If you break it it won't be covered under warranty.
It isn't unusual for it to take 20 minutes, it more, to completely drain the tank. You're talking about 40 gallons our more through a relatively small diameter tube that is only gravity fed.