Solar panel weather-resistance w/ permanent flat mounting

You could cover with plexiglass to allow sun but reduce standing water.
Or tilt the RV a bit to exaggerate the slope that you already have. Tilt toward the sun for best results. Possibly update the mounts to allow tilting.

Anyway put some water up there and see how much pooling you get. Maybe even a rag in the corner would wick away most of the water before it froze.

Or maybe your dad's panels had a defect or no bypass diodes?