2002 Holiday Rambler 2nd rear black tank

Just thinking through the problem with you..... If it was my rig, and I could not find a dump valve for the second tank, I would disconnect the external water source, turn off the internal fresh water pump so that no water would flow when I opened the toilet valve, hook up the sewer hose to a dump/sewer cleanout, open the sewer dump valve and let anything that will come out drain thoroughly, pour a pitcher or two of water into the affected toilet, open its flush valve to let the water go out and see if any water came out of the sewer hose outside. That will tell you if there is any connection between the 2nd tank and the primary black tank dump valve. If water DOES come out, then you probably just need to do a really good flushing of that second tank. BUT...I wouldn't try to do any flushing until I had determined that it was connected to the an open sewer valve. If you put water IN and it doesn't come out anywhere after doing the above, then yes, I would say remove the tank to see what's going on.