The Moss-Magnuson Act
(FTC.gov) forbids manufacturers from denying warranty service simply because you modified your property. From that link, "... The FTC says the manufacturer or dealer must show that the aftermarket or recycled part caused the need for repairs before denying warranty coverage. ..."
That said, it could lead to hassles. In my opinion you should be most careful about changes that might result in leaks. You'd want to, anyway, for your own benefit, but sources of leaks can be hard to detect. If it's anywhere near your modification that might conceivably cause water penetration, it might be hard to defend against.