tommymsw-
The plumbing diagram for you 2000 Adventurer 35B is at
this link. It shows the valves are immediately behind the sewer "Y" outlet. You may be able to reach them to open by hand.
If you can do that, then it's not difficult to reattach the cable to the valve stem. The cable is held into the stem by a small setscrew that has come loose or fallen out. Insert cable in the stem, tighten setscrew- done!
There's a wealth of information for your coach available in the "Manuals and Diagrams" section of this Winnebago page:
Winnebago Product Information