[JandC]

I think you can get just a gate valve that would attach to your exterior sewer boom. Instead of that I attached a Flush King to my sewer boom. It has a gate valve plus the garden hose attachment for back flushing tanks. The one I installed has a small clear section also which I find helpful when flushing tanks.

I just drilled two very small holes and installed screws through the overlap so I can keep it in place all the time.