To Tarp or Not to Tarp
If your roof is maintained in good condition, it will withstand the forces of Mother Nature without any extra protection. Consider how many RV's are on the dealers lot and none of them are covered!
As a concerned RV Technician, I am totally against the tarping of an RV with your hardware store variety plastic tarp. A plastic tarp laid on the roof and draped down the walls, is an invitation to dry rot - you want the moisture to escape - not to be held in, as a plastic tarp will do.
If you feel you need to cover your RV then a specially designed cover or shelter is the way to go. The cover is made of breathable material that protects your RV while providing the needed ventilation and airflow.