Ron, again with the slide out? Unless your slide leaks, which has not been indicated, it is NOT your main source of humidity. When our rig is stored, the slide is in and those heavy rubber seals snug it all up. Severe storms don't push water in through it so why would general humid air get in there? Every issue on this forum is not because people choose to have a slide.
There is some level of air exchange in every dwelling or vehicle. Since the outside air is humid, some of that humidity will sneak into the rig. You will flush in the humidity every time you open the door. My wild guess puts general incursion more on the truck area, around the house door, and through the roof vents. Those three areas have air leakage. As for getting humidity out, I agree that the air will equalize so putting a dehumidifier in the shower or kitchen sink should work. I'd prop the shower door open if you go that route.
If this thread prompts you to check for air leaks, a simple method is to close everything, light a stick of incense, turn one fan on high to exhaust air, Then move your incense around slowly and watch the smoke. You'll want to "outline" the windows, slide, other vents, and any areas you know or think there is an opening through the house "envelope" to the outside. This includes utilities, drain holes, etc.
