Could be almost anything. Is this a new rv?
Since it's on the ceiling, it could be water damage, but if you're in a dry area and had not rain, that's not it.
Could be that the glue is failing due to the heat. If you could store it in the shade, that would help.
If you think it's moisture from the air, get a couple of buckets of DampRid and close the camper up completely to store. DampRid is environmentally safe and the pellets will turn to water as it absorbs moisture.
Welcome to the forum.
2015 Dynamax REV 24TB class C