Atwood water heaters don't need an anode rod because the tank is aluminum, not steel. Instead of replacing the anode rod every now and again, you get to replace the nylon drain plug every now and again after it gets rounded off when putting it in and out, at least in my experience. (It's instructive that they come as replacement parts in in two-packs.)
That adapter for winterizing should do the trick. It's not hard to piece together a sturdier and more "official" looking version by getting a couple of plumbing pipe adapters at the hardware store and screwing them all together. As I recall, I ended up putting together a garden hose to 3/4" NPT adpater, a 3/4" NPT to 1/2" NPT reducer, and the 1/2" air tool quick connect.