I have found that by emptying both tanks the rear tank will just empty toward the shower tank and then the shower tank empties in the drain, or prevent the lower shower tank to empty, only after the kitchen tank. The kitchen tank is higher by the axles then the shower tank that is under the black tank, on the lower part of the frame on the 09's.
Maybe by opening both valves the higher tank will empty in the lower tank and push what is blocking it.
Close the hose connection with the cap and then open both tank. the higher (kitchen Tank) will fill the lower (shower tank) and then the blockage might just flush out.
Then close both valves and then connect the hose and try to empty the lower shower tank, and see how it flows.
I found this out by luck. I opened the both valves one time and the shower tank filled as per the gauge, and the kitchen tank emptied on the gauge. I was trying to fill the kitchen tank with the shower tank because we have a washer on it. So it didn't work just the reverse happened.