Many times, if the cable makes a tight curve, it will be difficult to move. So redirecting the cable (straighter) may help free up the movement. Another thought would be to cut the corner out of a ziplock bag. Then thread the cable end through the cut out corner. Next take a zip tie and pull it tight around the corner of the baggie and the cable, to seal it around the cable. Then put some motor oil into a zip lock bag, and position the bag/cable so the oil drips into the outer cable and down through the length of it. Work the cable back and forth a dozen or so times. Let it sit over night. That will lube the cable. In some models of trailers you can do this without needing to go into the underbelly.
The oil in the tank idea, is probably the best way to lube the seals inside of the valve.
