We keep ours outside, but parked by some of our big trees such that it gets direct sun only a few hours each day in the summer and almost no sun in the winter. However, the most critical part of how we store ours is we keep in it parked so that the overhead cab window never gets any sun.
It's the sun that's the worst enemy of RV exteriors. It not only fades colors - which can be slowed down with good waxing habits - but most importantly it drys out, hardens, and cracks sealants around all side and roof openings.
Humidity is the worst enemy of RV interiors. If you store it anyway other than inside in a heated room - it's important to keep the inside of the RV ventilated and, if possible, heated somewhat using a small heater set on on a low setting. The small air circulating heaters used in cabin boats when stored moored at a dock are good for this. We keep both rooftop vents open all the way (they have roof covers over them) during the winter rains and a small electric heater turned on during the rainy season. The coach interior should be kept ventilated and warm. We once almost ruined a motorhome by keeping it all closed up and unheated during cold, wet winters - mold formed everywhere inside. That's one thing that really concerns me about those complete canvas covers for RVs. They keep the sun's UV rays from ruining the outside ... but they also prevent interior air circulation and interior heating during moist winters. They trap moisture all over the RV inside and out so mold can form inside and out.
Keeping your rig under a carport, with roof vents open and a small heater inside during the winter is probably the best way to store it other than the very expensive way of inside a heated space. A small heater can be set on a moderater setting and plugged into an inexpensive mechancal timer so that it only comes on nights during cold and wet seasons.