repairs

I did something similar to Iamichabod's video except I just drilled a pilot hole and installed a grease fittings one for black and one for gray. Every couple of years I just pump some grease into valves and work the handle back and forth and they become easy to open again. For some reason the black tank dump valve is the one that needs more lube than the gray.

I really don't think they need cleaning but lubricated yes.

I have used this method for many years to lubricate my valves and have had no problems with them at all.

This Video will show how to do it

Where are they located?

If they are right at the outlet, it's pretty simple. 4 bolts and the come apart (make sure to fully drain the tanks of course before pulling the bolts).

If they are remote (operated by cable), it can be more challenging getting to them (often they are hidden under the underbelly covering with no easy access) but once you have access it's not greatly different.

While I wouldn't discourage lubing them, regular use is probably the best way to keep them operational.