Thank everyone for your comments.
Additional comments: To clarify my first post--when I replaced both valves, it is extremely tight. I can get one on, but when I start on the second one I have to push the entire T up in place. This is when I feel I am damaging the gasket. I have a flush system that came with the camper. I connect a hose to an outlet on the camper, turn the water on, open the black water valve and it works like a washing machine. So, the black tank gets emptied. I go home and the next time I go camping and hook up the sewer hose, some stinky brown water comes out. I feel sure it is the black water tank and not the grey water tank. Same thing happened last weekend. I went camping, really flushed it out till clear water was seen and went home. Today, I went to the barn to check it out. A small amount had leaked past the valve.
I have a picture, but have not figured out how to post it.
Thanks again,
JP