By your list, it looks like you are doing it right. Blame it on the rain. It probably never had a good chance to air out from the previous night(s)moisture buildup. We had a family of 4, with dogs in our 19, Orbit in temps colder than that, but with fair weather and only had a moisture problem under the mattress.Everything else was completely dry.
BTW, The sun sure helps in drying things out, so we avoided really shady sites when winter camping. During the day we would even run the AC to help on humidity at times. With the sun beating on the HTT, it used to get really hot in there so it would make a good excuse to exercise the AC and help dry things out when there was no breeze on that particular day.
