Replacing the tub will be an adventure in futility! Campers are built from the inside out. Tubs and showers are usually the first thing laid on the floor and then everything is built around it, opposite like you'd do in your house. Here's a photo of a Denali 5er from a tour we took of the Dutchmen factory in 2006. The manufacturing methods have not changed. Maybe this photo will give you an idea of how the tub is installed.
But really, if you patch it with one of the suggestions above and it works, it will be much easier and cheaper.