As long as the windows
don't let water get in from rain or hoses, I'd go with more versus less ventilation. Nothing gets stale faster than closed up areas.
Also--are you using an air-drying product like Dri-Z-Air (I can never spell it)? You can get it at many hardware stores OR you can get a $1 version at The Dollar Tree. They both work to keep the inside dry and smelling better.
And be sure that you check inside periodically (at least once a month?) for any leaks
or other issues such as mice or wasps. A recent problem is easier to deal with than a long-standing one.
PS We don't cover ours, but so far our home-done marine paint
exterior is holding up okay. If we had a nice gel-coat, who knows?