I read on one of the valve MFG'r websites that you are supposed to lube the valves by dumping 1/2 gallon of coconut oil into the empty holding tank once a year to lube them.
Yes 1/2 gallon!
I know this because I had to buy a new valve after mine broke, it was very stiff compared to the new one.
So I do believe they need to be lubed.
Keith