One of the two gray gate valves( bathroom gray water) in my 2014 Reflections 303RLS is stuck open. When you push the handle closed you feel a pushback almost like something rubbery is in the gate. Maybe one of it's own seals? I have two questions.

Can anybody show me pictures or describe in detail where to cut the coroplast so as to make the access hole as small as possible with enough room to complete the replacement?

Has anybody replaced this valve? If so any helpful pictures and information would be appreciated as to performing the replacement and ordering the parts.

Thanks in advance, Jon