Yours is not an unusual complaint. If the coach is level and water blows in under the slide topper (or if you have no topper), there will likely be a thin pool of water on top. I would drop (or raise) either the front or the rear of the coach a couple inches if I suspected that, causing the water to drain to one end or the other. And bringing it in several inches and waiting a bit, as dons mentioned, also helps.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL