That question is the MILLION DOLLAR question.
So, I'd suggest this (or you'll make yourself crazy). Start by reading the winterizing section of your owners manual. THEN, I'd go ask a local RV dealership for your AREA what they recommend.
We have researched that information out for a couple months now, and it REALLY REALLY varies according to where you live. We were 100% convinced not to cover our TT this winter, and then out of the blue someone who lives in our vicinity posted that if not for his TT cover the top of his TT would be destroyed from all our falling pine needles, and smaller branches. We are in heavily wooded areas that can be prone to high winds. He's had no mold problems etc.
We have made ourselves crazy with this topic...lol Of things we have decided on - other then the full winterizing suggested in the owners manual for our TT is to put olive oil in the toilet bowl as it won't freeze, but it will keep the toilet ball seal from drying out over the winter. We ARE going to shut windows/vents up tight but LEAVE ALL CURTAINS UP so what little sunshine we see in the GREY winters of the PNW can hopefully thwart the mold. Also, bought a dehumidifier and will run that in it because of our high humidity. I may contact the people we bought it from too. They were locals and this 7 yr old TT was mold free,and spotless when we bought it.