Hi Chris and Mia, welcome to FiberglassRV
I'm one of those folks that believes in starting simple (and cheap) and moving to harder and more expensive.
So... what makes you believe there's something "rotten" behind the ensolite? If what you see on the surface... that can be cleaned successfully! And cheaply. Removing the ensolite and trying to replace it will be expensive and a whole bunch of work...
Pictures?