I am no expert on RVing but I do know that the city water coming into my house comes from a huge tank that sits out in the Florida sun every day. In the summer, it's even hot coming out of the cold tap.
Seems to me that IF you maintain the system religiously, the water in an RV tank should be safe. If you ignore it and leave the water in there stagnant for weeks then no. See this link. It shows that the treated water is stored above ground in the sun and then comes straight into your house. I don't see the difference since an RV tank is just a storage tank.
http://water.epa.gov/learn/kids/drinkingwater/watertreatmentplant_index.cfm