Cajun,
I would have thought it would take longer from 85 - 90 also, but it did not. I think it was a constant 2 - 3 degrees an hour on average. Heating during the day (direct sun) was a big help. I wouldn't know about letting cool versus keeping it at temp. I would be too worried to run out of gas. If I had a NG line, I might try to keep at temp or at least a baseline of 82.