Charles, I'm interested in that windshield cover as well. This was our first time camping in hot weather and you are completely right. As soon as we parked, the heat started flowing from the forward area, very noticeable when the curtain was put up (which also does not give full coverage- a little more velcro and material would make a big difference). So the difference is easily felt? Then sign me up!
I have that 'this is not storage' area under my sink, where the HWH is. (btw, we didn't get the oven either-more room for pots/pans) There is, or should be, no battery there. You might be confusing it with an electrical panel/transformer? Batteries must be outside the coach.