I have had good luck with Bruce's Covers for several different planes. Solid fabric and good mechanics. Good phone service, but maybe some production time before delivery.
When I was at risk of losing my hangar space (due to a priority Pilatus that was going to empty our shared hangar) I found out that Bruces has an extended cover that extends the sides down onto the wing root by about 6 inches out on the wing. Costs a bit more. Turns out that the extended canopy cover is one part of the full canopy/wing cover system that they sell. (Fortunately, I never ended up in the tiedowns.)
The extension onto the wing reduces the possibility of wind lift and seems to eliminate rain and melting snow from entering the wing root. I think that will solve your issue. I used to live in Virginia, and know you don't have the high temps/scorching sun and/or heavy snows for which I would recommend the complete cover. I see they have several choices for the RV-12, including an extended cover. https://www.aircraftcovers.com/RV12