The first thing you need to verify is that there are absolutely zero water leaks in the coach. Roof, roof components, windows, doors, compartment doors; literally everything screwed or attached to any exterior surface must be verified to be leak-free.
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Likewise, the plumbing systems must not have any leakage. I do recommend blowing out all the fresh water plumbing lines, thereby rendering the entire fresh system void of water. Keep water in the toilet bowl, the P-traps and in the bottom of each holding tank, however.
Another key to summer storage as opposed to winter storage; in the winter, I like to see the coach completely air-tight, but during the hot, humid months of summer, I like to see some airflow through the coach. If you have a powered vent or similar cover over the 14-inch roof vents, leave one of them open at one end of the coach and leave a window cracked open at the other end of the coach; preferably a window closest to the floor, so convection air can flow through the interior. Now I know it will rain, so protect that window opening with some plastic sheeting or something else that can absorb any sideways raindrops that might enter. This is the toughest part, obviously, but doable.
Another option is a total coach cover. A custom one that allows breathing to take place is preferred. With a custom cover, you’ll be able to vent air through the open roof vent and draw in fresh air through that window without the fear of rainwater entering in. Aftermarket desiccant containers are readily available from many retailers. They will absorb the moisture that will indeed enter the rig. Place them on a pie tin at various locations throughout the rig. I’d rather deal with collecting the moisture than tightly sealing the coach. A hot, stuffy, humid interior will further the proliferation of mold faster than one with ventilation, even though moisture will undoubtedly enter. Heat is one of the biggest generators of mold. The desiccant granules will absorb the excess moisture. You may need three or four containers, depending on the overall interior length of the RV. But in my opinion, a ventilated coach will be a cooler environment and less prone to mold development.
Yes, cover the windows! I recommend half-inch thick sections of styrofoam be cut for each window opening. They can be form fitted or taped in place. Be sure to treat and cover the tires as well. Also, bring all the slides to the retracted position. You should be just fine come December.