After the winter in storage the rods pull but require more effort than previously. What product is best to use?
The issue usually isn't the rods dragging, but the valve faces. Some folks use vegetable oil and warm water in the tanks to lube the faces "from the inside out".
Vegetable Oil to lubricate Black and Gray water tank valves (https://www.foreforums.com/index.php?topic=12621.0)
The alternative involves disassembling the valves to clean, lube, and reinstall. (or install new ones)
There was a post one time (from our friend nitehawk) about drilling a hole in the valve body housing, then injecting silicon grease that would lubricate the sliding part of the valve AND the rod........
.....................(searching)...........found it:
Sticky dump valve? (https://www.foreforums.com/index.php?topic=46947.0)
https://www.youtube.com/watch?v=TGv9THpOdmw
Empty tank, close valves, pour quart of cheap vegetable oil into each tank, let it settle to bottom, after a while, quick partial open dump valve several times. Works very well.