Another winterizing question/ plumbing

We had TT for 2 or 3 years. First year we couldn't go south, so this is first time winterizing this RV.

Suburban 6 gallon water heater. Only one (1) knob that I could find on cold water side. And I looked real good. And looked again to be sure after the Easter egg hunt in all the insulation for low point valve drains. Could not find valve for hot water side. When I put air to system no air came out of hole where anode goes. So something is in system.

So must be a check valve? (not the check valve in city water flange) but another one?

Anybody have any insight? 2013 Arctic Fox

Thanks