You should be able to locate the valve, especially since it is attached to the handle that you use to open it.
Try pushing in on the handle and then pull out on the handle. You may have to use a pry bar to give you a little leverage. Don't pry too hard. If you can get it to move in or out just a little bit, you are on your way to getting it to open. Just keep working it back and forth. It might help if you poured a gallon of water down the toilet to get things wet on the back side of the valve.
I used this back and forth method to free up both the grey and black water valves on my 38 year old trailer. It worked for me.
Knowledge: "A gift to be shared. A treasure to receive."