Welcome to the forum!
If your rig is like most, the blade valve is pretty easy to replace and they only cost about $15 so replacing black and grey at the same time is prudent.
Hopefully.... your current valve is still working and you can get both tanks dumped. Then you might want to use your jacks to tilt the coach toward the drains to get a better drain. Then, tilt it back the other way a bit so you stay dry during the job.
They come apart by removing several bolts on each and then just working the parts loose until they separate. I like to put some lithium grease on the blades and rod on the new valves before installing.
Rick, Nancy, Peanut & Lola our Westie Dogs & Bailey the Sheltie.
2007 Itasca Ellipse 40FD