I'd rather be sailing
Case in point - we haven't used our pumpout deck "fill" since late February as we've always been places that we could go directly overboard. Now that we're back in inland waters we went to remove the cap and it wouldn't unscrew. We tried everything (except for heat since it's filled with methane). Finally, we pulled the whole old unit off and put a new unit in. We could then heat the old unit red hot, and only once we used a sledgehammer could we pull the cap off of it. What did we find? Apparently, it was 304 stainless, or something of the sort (it was magnetic) and the threads were completely corroded. If we had pulled it off every so often, we would have probably been OK, but we didn't. So... if you have deck fill plates that you don't use regularly - use them or lose them!