Been there done that. Replaced it twice, and still leaked.
Turns out it was leaking from a 'hairline' crack around the tabs on the sewer line where you screw the valve in.
Checked the gray tank valve and there were hairline cracks around the screw tabs there also.
RV repair says they see this a lot because if you look at your sewer line assembly it is suspended and it moves up and down and the movement of the RV puts stress on the screw points.
The hairline cracks were not easy to see but that is where it was leaking from. When we removed the valve the second time after seeing the cracks, 2 of the screw tabs actually broke clean off.