Welcome to JOF!
I have a thinner topper, but don't fold it inside the mattress. I fold each separately and stack then and they fit the space easily. On our 19H there would be room for a thicker topper if we bought one.
We camp in Canada in temps down to and below freezing. Have never had a problem with condensation, but we keep the space properly ventilated. I would wait and see if you have issues before buying hypervent, you might not need it.
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2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2018 Nissan Titan Midnight Ed.
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