Over the last 8 years or so I have made access holes in the Coroplast underbelly for every valve and access point there is !
I have found Gorilla tape the best tape for this job. If you clean the Coroplast properly the tape holds perfectly and when you need to gain access again and peel it off with no issues, no sticky mess from roofing tape.
If I ever get the urge I will section off the Coroplast so I can take off and replace the panels when needed, no tape just some screws or bolts. I'm thinking 3 panels should do it for my camper.
Ron W.