You're probably better off just replacing the dump valves. Once gunk that's on the slide valve and in the groove hardens, any lubrication you put on it will just be temporary.
It's not that hard - make sure the tanks are empty, then remove the 4 nuts and bolts on the corners of the valves and spread the plumbing slightly.
When you put in the replacements, keep them wet. After you dump pour some water down the toilet and a sink drain, enough to fill the outlet pipes. This keeps the gunk that inevitably builds up on the valves soft and pliable so the valves work as they should. If you winterize the rig, pour some antifreeze down the toilet and drains to keep them wet over winter.