The best solution that I have found for humidity is to allow it to ventilate. I have vent covers in the bedroom skylight in front and bathroom skylight in the back. I leave both of them cracked open about an inch, which allows fresh air to flow through. Before I did this, I would have that damp smell in the camper. Since I started doing this, no more smell.
Bugs are harder. They can sneak into the basement compartments, which are rarely completely sealed from the interior of the RV. I would clean it really well when you are putting it in storage and get rid of any food scents. That will make your RV a less likely target for bugs.
I just keep my trailer plugged in year round (but I leave it in the backyard). Keeping it plugged in keeps the battery charged. If you don't have that option, I might suggest a solar battery trickle charger.
I always make sure to drain my water heater and open my low point drains when I am driving back from the campground to completely drain the water out of the camper. One time I forgot to drain my water heater and got that sulphur smell from bacteria in the water.
Hope this helps!
