First, anything exposed to sunlight, rain, snow, heat, cold, and so forth (particularly UV and moisture) will deteriorate with time. Look at all the old run down RV's you see sitting out in the middle of some farm. Those things aren't rotten to the core because they sat in a well protected shelter for 40 years. They're rotten because the sun's UV rays deteriorated their outer skin, and because water intruded and ruined the structure. Rubber gaskets, fiberglass, paint... everything breaks down with time, and the break-down is accelerated by what we call "the elements."
Second, Forest River's own manuals recommend storing away from the elements, and recommend that if indoor storage can't happen, a cover is the next best option.
Even if you don't plan to keep it very long, keeping it covered will preserve its clean appearance, and help with resale value down the road. And if you do keep it a long time, protecting it from the elements will reduce the likelihood of developing leaks, cracks, faded decals, faded or worn out paint, etc.