Thanks to all of you who responded with advice and experiences. Problem solved. First, the sensors are not correct. Secondly, the valve at the sewer line outlet
is the black water valve, the only black water valve. The handle forward of the sewer line is for the front grey tank - the cable was loose and the tank was empty though it was reading 2/3 full. It took dropping the belly pan to determine that.
These units were built with two pull handles side by side forward of the sewer line, one for the grey (grey tank 1) and one for black water tank as the diagram in post #23 shows, with the galley tank (grey tank 2) pull handle just forward of the front wheel. Apparently the original owner had that system modified because the hole through the belly pan where the black tank pull rod would be has been repaired with a small black tarry type patch. A Cedar Creek service rep told me that that's what they use to patch the underbelly. So, now I guess we can shower...it's been days and days. lol
Thank you again.