[Carl n Susan]

If you are minimally comfortable with tools, gaining access to the black tank valve is easy. There are several threads here about it. Otherwise you can cut a access flap in the coroplast to access the valve. The quick solution, regardless of the access technique, it undo the cable at the valve and open it by hand.

Replacing a cable or even the whole valve assembly is relatively easy. Order the Valterra kit (don't mess with the other brands) from Amazon, drop the coroplast, and change the valve. It should take no more than two hours from start to finish.