Yep, that is where it is on my 86. Like most of us when laying on the ground, looking up in the wheel well, and you go to turn the plastic handle, the valve is going to break. Cuss, cuss. Be prepared to spend the rest of the weekend replacing it. There will be an access port under the fresh water tank that can be removed so you can see where the valve is screwed into the plastic tank. This whole mechanism was not one of Airstreams better ideas.
Our you could just turn on your 12v
pump, open a faucet or two, hold the pedal down on the toilet, and drain the fresh water into the gray and black tanks. Let it run until very little water is coming out the faucets. And then pour a gallon of RV antifreeze into the fresh water tank and call that area good. Of course there is a lot more to winterizing a trailer than just the fresh water tank.
Hope my bad experience helps you avoid one.