I'm thinking not that lucky!!
Looking at the plumbing drawings for your RV here:
https://www.winnebago.com/Files/File...f35a_plumb.pdf
Not spotting the exact lines, etc that show in your picture but I do spot a couple that look near the same and I "think" both are drains for the tanks.
Maybe take a look at where your tanks set compared to the drain you have to check my thinking!
First thought was the valve was too sturdy for the flimsy plastic we often find but these look like good stuff on the drawings for that year?? Maybe just better than I expected!
I'm thinking that valve is about level with the tank so it would not drain up and over to the valve?
A thought on trying the pop off/ pressure valve on the heater?
If there was no pressure on the lines from hoolup or pump running, that would explain no water coming out as it is near the top.
Before trusting, I would pull the plug to verify before freezing!
Do you have the manual for locating the drains? Not very specific but a start on searching for them as well as looking on the drawings for valves.
Good luck and shout back if we can help.