Butyl should be fine.
Your post reminds me of our old fiver - the manufacturer cut a square hole, but then filled it with a square WH with rounded corners. While the facia covered the entire hole, there wasn't sufficient contact at the 4 corners to create a seal. I found this out by removing the wrap-around seats & seat panels at the rear of the fiver while working on other projects, and found the floor was soft under the seats. Upon chasing that, I found the source was this manufacturing defect, which allowed rain/wash water into the unit which flooded the floor. Because the floor vinyl wasn't sealed at the floor/wall intersections, water soaked into the particleboard flooring over preceding 7ish years before I purchased it. I replaced stringers and the subfloor in the rear 1/4 of that unit, and installed new vinyl which curled up the walls by an inch or so to prevent something like that from happening in the future.
To fix the issue, I added corner blocks to the opening, then reinstalled the WH with a properly sealed facia/face plate. I used butyl tape, then sealed any small openings in the face plate with silicone.
Several years later, leak free, we traded the fiver in on our first class C.
2016 Forest River Sunseeker 2250SLEC Chevrolet 6.0L