RV covers are NOT designed to keep the moisture out. They breathe, which means the moisture will come through it and then evaporate. A tarp will just collect condensation underneath it and not let it escape. A cover does a better job of keeping dust, debris and UV rays off of your RV. 90% of the tome, the problems with covers is improper use. They need to be properly tightened on the coach to prevent billowing, which causes the cover to move on the coach and rub holes through it.
FWIW, my trailer is a 98 and has never been covered.