I think we've solved the problem. It wasn't the problem I thought it was, but we tracked it down.
I wasn't too worried about being able to fix the valve once I found it, but where's Waldo? I got a response from Grant at Bigfoot and learned the square cover I'd found was the access panel for the black valve. It is located on the drivers side over the front axle
,on the 25RQ. I removed it this morning and lo and behold there was the black valve as large as life.
I used the knowledge gained from U-Tube to pull the valve cable mount. It was attached and looked OK. It seemed the knife blade was moving in and out, the valve seemed to work. At that point we thought it might be a plugged pipe. Ran a hose with nozzle up the sewer outlet till it hit the valve, opened the valve and pushed the hose onward, proving the valve works. We spent some time with the hose and nozzle along with a plumbers snake. All the while catching flow in a bucket. Must have worked slowly 'cuz there was no gush of "fluid" from the tank. But we did get many buckets full. Finally looked down the toilet and the tank seemed empty. So we filled it about 1/3 using a cleaning wand. This dumped as it should, so we are a success. Put the panel back and we're ready to go. Thankfully I didn't have to remove the valve with a partially full tank, now that would have made for a bad day.
I did learn, for sure, that the black tank flush is not functioning. But we can live without that for some time.
Thank you for the insight and help.
Life is good.