Double roof vents don’t help with humidity, ventilation in general doesn’t really help with humidity. I’m a big fan of window vans, but even with all those great opening windows, they don’t really help with humidity.
Dry heat combined with ventilation can work in cold weather, in hot weather you either need an air conditioner, or an indirect type swamp cooler. An air conditioner requires either shore power or a generator, but a swamp cooler can easily be run off your house battery. Mine uses about 3 amps on high, maybe about 2 amps on low. It can drop the indoor humidity by about 50%, and drop the temperatures to where they’re comfortable. They do require refilling with water, but in my case water is a lot easier to come by than shore power for a regular air conditioner. I don’t like running my generator that much, or paying to stay in parks with power hookups.
To get a dehumidifier large enough to have a chance of working would surely require shore power as well, I would probably opt for a window air conditioner before a dehumidifier though, because the price is likely to be similar. You could mount a small window shaker in a front window once you’re parked. I built my swamp cooler to sit on the front passenger seat and vent out the front passenger window. That way nothing is hanging outside, and I can also use it while driving, since my van doesn’t have A/C.
If you do have factory A/C, you can try running it and parking in the shade to both cool and dehumidfy your van. It this case you would close up all ventilation to keep the humidity/moisture out. If you’re parked in the shade, the cooling effect of your A/C should last for a while. If I’m parked in the shade, I only need to run my swamp cooler about 15-20 minutes per hour to keep things comfortable.
You will likely have to build your own indirect swamp cooler if that’s the direction you choose. Overall, it will likely cost less than a cheap new air conditioner. You DO NOT want a direct type swamp cooler, because they don’t work well in high humidity areas. While they are still better than nothing, an indirect type will work much better.
In the meantime, get a dollar store trigger spray bottle, fill it with water, sit in front of a fan, and spritz yourself as needed. They also have cooling towels and wraps that you soak in water, and they’ll last for several hours between soakings. Sporting goods stores should have them. Construction workers and sometimes flaggers have cooling vests and other solutions to beat the heat as well. Google might even have more ideas. Beating the heat and the humidity is possible if moving to a more comfortable climate either isn’t convenient or possible.
Some people also hang out in shopping malls, libraries, or anywhere there’s A/C during the heat of the day, and only return to their rigs after it has started cooling off for the night. Coastal areas, and anywhere near water can also be cooler and breezier as well, but they can also be buggier if there isn’t a good breeze.
Hope these tips help…