Malcolm-
The plumbing diagram for your coach is
here. According to that diagram, the valves and cables are directly behind the white panel that holds the valve handles. You should be able to see under the panel, or reach under the panel, to figure out what is not working. It's possible that the cable has pulled free, and just needs to be put back in place and the setscrew that holds it tightened.
If you have to replace a valve, then it should be pretty easy to do. Drain both tanks dry. Unbolt the y-pipe at each valve. Remove the y-pipe. Repair or replace valves as needed.
There was no "main shutoff" as it came from the factory. It sounds as if a someone had a leak in a valve, and instead of fixing it he stuck another valve on the y-pipe outlet. Better to fix the leak, if that is the case, and get rid of the "main shutoff" valve.