I have never really checked but when I hook up my 12VDC non-tank Compressor to the trailer city water inlet to blow out the water lines I empty the water heater pretty much as well by opening the hot water valves at the sinks. My hot water cold water input is on the bottom of the water heater. If there is any water left inside the hot water heater it will have alot of expansion room if things freeze up. After I drain out all of the water at the low point and blow out the lines then I add some pink stuff to the P-TRAPS.
Note this typical water flow chart for the trailers.

I probably am flirting with death but I have never drained my water heater at the bottom port for winterizing. I have done some maintenance of flushing the water heater by removing the bottom port and also removing the water heater rod anode for inspection. I do sometimes here at the house open the top spring loaded pressure relief valve over the winter months.
Our winter months here in Northern Neck VA are very mild. We probably have only had a couple of nights of freezing overnight and it always warms up above freezing the next day. This year may prove different since the whole country is experiencing strange weather patterns...
Roy Ken