About 4 years ago I noticed a small crack in the grey tanks dump handle on our 24' Class C. It still worked but the coach was definitely giving me a warning/heads up. I went on line {Amazon } and ordered an entire new dump valve assembly {for both grey and black - if one is going bad rest assured the other will not be far behind} for about $100.
I was able to do the replacement at home with all of my tools vs stuck somewhere on the road if I was even lucky enough to be able to acquire the requisite parts. There was more reaching under the side of the coach vs crawling under It took about an hour and half
and was fairly easy to do, just reverse engineered the removal of the old valves.
Now picture reaching for that valve and having it break off in my hand at a crowded dump station... :S Order the parts and do it yourself or ask a buddy if you have one with more expertise than you. As noted above look for any RV repair shop to be backed up weeks/months and about three times the cost of doing it yourself.
Good luck.
:C