I noticed that you have a Columbus 320RS. Many of them have a problem with the black tank valve cable being too long and not properly secured, this causes the cable to flex excessively when operating the valve and binds. Most have had success in securing the cable to the frame or brackets in the basement using nylon tie straps along the length of the cable. Some have shortened the cable, too. Lubing the cable is still a good idea, though.
2013 Palomino Columbus 320RS
2007 Dodge RAM 2500 6.7 Cummins
Reese 15K manual slider
M.I.L. in Florida for good!!