On mine it will drain a way down if I open the HOT WATER spigot on the sink when draining from the low water point under the trailer.
If I want to get all of the water out I remove the bottom anode port which is my hot water heater sacrificial anode and give it a good inspection at the same time. when it is all eat up you need to replace it... My hot water heater is a suburban model... The ATWOOD models don't have one I think...
Getting all of the water out is not all necessary if you are worried about freezing over the winter months. There is alot of expansion room in the 6-gallon heater case. I always lock open my hot water release valve on top during the winter months which also helps draining most of the water out using the low water point on the trailer...
Roy Ken