How often do I have to change out the water in the fresh water tank

This would be a personal preference. There are a few factors that will play into what you decide. One factor is it depends how often you will be camping and how long the water is going to sit. With any type of moisture over the coarse of an extended period of time mildew and mold can begin to set in. You must however during the off season winterize your unit or you will void the warranty of your camper and cause plumbing, fitting and appliance issues. Cosult your manual.