The discussion on RV covers can get very heated. Many people don't get much more than a year or two out of a very expensive cover before it wears through on the corners and starts to tear. When that happens, it's only a matter of time before the cover is in tatters.
The problem with tarps is they don't breathe, and some believe that contributes to condensation on the inside of the tarp, which then ends up causing damage to the camper. Also the blue tarps degrade quickly in UV light, and have the same "wear on the corners" problem as RV covers.
I've heard of some individuals going through efforts to pad the corners of their RVs, and/or build frameworks to allow for airflow under a tarp. Seems like a lot of extra work to me but if it works for you...