Mine did the same thing a couple of years ago.
I drained the tank and left the valve open and the cap off for a couple of weeks and then I got down and dirty and removed the valve.. It was dry by that time, so not bad at all..
Turned out some TP had gotten wedged in the gasket where the blade slides in and that caused it to not seal completely.
After cleaning the gasket and putting it all back together, 100% sealed again..
Cost was zero $$ and about 30 minutes of time, which was mostly prep to crawl under the trailer in the first place...
Now I just have to fix the broken rod handle to the gray tank.... ;) (I can still grab what's left of the rod and pull and drain, so it's not on top of the list right now.. )
Mitch