Thanks for the great advise. Really makes sense.
I found this article that compares compressor vs thermo-electric dehumidifiers.
Thermo-Electric Dehumidifier Reviews | How They Work
I prefer not to leave the van plugged into 110v power full time. I have been plugging in for a few days every three week just to trickle charge the flooded acid house batteries. I am leaning towards going with a higher capacity compressor dehumidifier and running it for a week or more, then unplugging and watching what happens. To your point, air humidity will probably rise, if for no other reason but the moisture in seat cushions and wall coverings. As it comes back up, I will turn it back on for as long as needed.
I suspect that I have some roof leaks, which need to be sealed but until they are, the humidity problem will recur. We don't get much rain here from March to October, so humidity will not be an issue during those months. By next fall, I will have resealed all roof perforations and there should be less of a problem.