difficult gate valve

The black water valve is really difficult to pull and push. So much so I'm afraid I'm going to break something working it. It is the cable system that runs from the handle back several feet under the trailer to the valve on the tank. What a ****pie system. My question is, is there a lubricant or any other ideas for this problem. I know there are some things that you can dump down the toilet to lube the valve but I would guess these would do very little good if any and would they lube the cable itself? rich