You would have to stay with the same opening diameter and hopefully have enough backspacing for the increased tank length. Plug around the atwood and suburban websites for dimensions based upon whichever model you currently have. Keep in mind all the plumbing at the rear would move back as well, including the bypass loop, and of course, any wiring.
94-Newmar Kountry Star 40-KSDP
Spartan/Cummins 8.3C-300HP/Allison 3060 WTEC-II/25yr RV Tech RVIA Certified/Onan-Cummins Certified