ktmrfs wrote:
run the dehumidifier discharge hose into the shower basin, leave the grey tank outside drain slightly open, problem solved.
And if the OP is concerned about pests flying up into the grey tank, he could leave it closed and just open it once or twice a season.
But I agree with another comment, I leave my roof vent open almost year round (has a vent cover to keep rain out) and leave one crank out window at least vented almost all year. I only close them in the spring when the pollen is ridiculous.
Our old camper (bought used) initially had a mildewy smell when it sat in storage. After we started keeping the roof and window vented open, the smell went away and never came back.