I changed the gray water valve on my Jayco earlier this summer. I had to split the underbelly covering to access the valve. When finished, I used Eternabond tape to seal the underbelly. We just got back from Alaska and the Eternabond held.
BTW, be sure the black tank is empty and thoroughly cleaned!
2003 GMC 2500HD 4x4 Duramax/Allison
2004 Jayco Jay Flight 25RKS
Equal-i-zer Hitch, 3502 Ultra Fab Tongue Jack
Twin Honda 2000i w/parallel kit