The valve is replaceable and should be no more the 3 or 4 feet form the outside. You can cut a large 3 sided square out of the plastic leaving the forward side intact to act as a hinge. Pull it down and you should find the valve. I'm assuming here that yours are cable pulls since you cant see them. It may be just inches away from the pull handles. The valve is held in place with 4 screws. Amazon is your friend. Be very sure the tank is empty first unless you are in possession of a hazmat suit!
Put the plastic back in place with Gorrila tape.
If that sound like a lot of work then just leave the one you have open and install a twist on gate valve where you place you sewer hose. Put a screw in it to keep it inplace.
All the sewer parts you will ever need.