Two fundamental differences between boats and RVs:
1. Boats are open to the weather up top. The wrapping is mostly to protect the interior from the vast majority of the weathering that would occur if otherwise left out in the... weather.
2. Boats are built to get water *in* them and have it drain out. RVs are not; they depend mostly on being able to shed the water quickly to prevent intrusion. If the water is held on the RV, sooner or later it will penetrate into something that is not water tolerant.