Ken, I'm also surprised that your GH doesn't heat up more than that in the winter; yesterday we had 28F outside and inside my hoophouse -- which is only covered with a layer of 3-mil plastic - it was up to 55F. Inside my Rion, which has the twin-wall poly, it was nearly 70; and the sun wasn't out for that long during the day. I've never actually tried using the water jugs -- I'd like to in the hoophouse, but there isn't room because the plants are growing in the ground and I only have about a 1-1/2-foot wide path down the center. (I obviously need a larger hoophouse!) After warming up yesterday, it was still 24F this morning, when it was 15F outside. Even in the middle of the winter, when the high outside might be 15F or so, if the sun is shining all day my GH will easily get over 80F.
Also, water actually gives off heat as it cools, and even as it freezes... that's how the "wall of water" things that you put around plants to protect them from freezing work.
Is it possible that the solar cover on your GH is actually preventing it from warming up more? That doesn't really seem logical to me, but maybe it's actually blocking enough of the sun to have that effect
