To clean the mildew, a bathroom mildew cleaner may help, depending on the surface, bleach water.
Yours is not the first thread about excessive moisture in an RV in the Pacific NW area. The weather there must be horrid this year. A few have posted about this problem recently.
Considering you are actually in the camper, have electricity, and actually using it, the option would be a second dehumidifier. Air flow is important, but if you crack a vent or window, your letting as much humidity IN as you are OUT. Heat will dry out the existing wetness on the walls and such, but the heat can also cause more condensations to.
I think your best option is a second dehumidifier and hope for an early dry Spring .... After all ... that darn groundhog in Pennsylvania did say it would be an early spring!