I first tried using machine oil to lube the cables. I used a piece of clear vinyl tubing that would slip over the threaded end at the top. The machine oil did slowly flow down the cable. However, it also quickly drained out the bottom end of the cable, so it didn't help.
Later tried the mineral oil to clean and lube the valve blade and seal. That definitely helped. I used it on the grey valve first. Then the black. With the grey tank, blow out the p-trap first, make sure the tank is empty, then pour the mineral oil into the p-trap followed by some water. Let sit for a few hours, then use a bucket to recycle the oil/water mix. Open and close the valve quickly a few times as the mixture is draining. Repeat a second time. Then do the same thing with the black tank valve.
I did this about 1 yr ago. The cable still requires a lot of effort to get it to begin to open, but once it starts moving, it becomes very ez, almost too ez.
My conclusion is that the cable is not the problem, but it is instead a sticking valve. Solids can build up on the seals/blade and even get behind the blade. But flex in the cable makes operating a sticking valve so problematic. Technically, blade valves should be installed with the blade vertical to minimize buildup. But practically they are mounted in the worst possible way, horizontal to slightly down.
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK