I used Silicone spray on the valves in the old trailer to keep the seals from sticking. They were readily accessible. The new trailer has them stuck way up in the covered "basement". IIRC somebody ...
I know I didn't believe it either till I tried it. Long time Rv'er told us about this. Next time you empty tank do a good back flush to get everything out the best you can and add a jug.
Get a bottle of valve lube stuff and feel it between your fingers then do the same with some type of vegetable oil. See if you can tell the difference.