There was a dehumidifier made for boat and RV use, no longer made. But you find them every so often on eBay. Search for "dh5 dehumidifier -hisense" on eBay, within a month or two one will likely come up. These are a small countertop real dehumidifier (not the cheap peltier devices you find now). It is arranged so that it will drain directly into the sink. They were made by Arrow, you will find them also in a SS cabinet (that was the marine version).
I wish that the RV suppliers would make a rooftop dehumidifier/vent to replace the AC unit. In a lot of climates, that would work very well and be capable of battery operation. 90 degree weather isn't very bad at all, if the humidity is <30%.