I had mentioned a couple of years ago that with my 19' Escape covered and stored away for the winter, the rooftop solar panel received sufficient sunlight during the winter such that the dual 6V batteries were fully charged when the trailer was uncovered in the spring (about 5 months later). There was a bit of skepticism shown on this forum on whether or not the solar panels could actually charge the batteries when covered up; but I never gave it much more thought.
I recently purchased a 15B in summer of 2017 and added rooftop solar panels to it just before winterizing the trailer and covering it with its Calmark cover for the winter. At the same time, I winterized my 19' Escape and covered it up with its own cover (http://www.escapeforum.org/forums/f1...er-9662-3.html
). The dual 6V batteries in both trailers were fully charged when the trailers were put away for the winter. Just last weekend, I needed to move the trailers around a bit in the yard to be out of the way of my yardwork, and while doing so I checked the state of charge in the batteries. Upon checking, batteries in the 19' (with the Expedition cover) were at a 100% state of charge, while the batteries in the 15B (with the much thicker Calmark cover) were at about an 83% state of charge. I plugged the 15B into AC power, and the next day the batteries showed a 100% charge. So it appears that the thinner covers do allow some sunlight through which can be used by your solar panels while in storage. The thicker Calmark cover does not appear to allow sufficient sunlight through to charge the batteries.