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Might consider keeping at least one vent on the roof: hot air rises.
Last I checked, MaxxAir is the only vendor with a vent which will operate when fully closed (crude ceiling fan), can operate while open and raining (without needing an external vent cover), and can leave the vent open while driving. So this is another thing to consider.
And given that this is looking like a re-engineering job, check out whether it is comfortably possible to sacrifice some overhead room (floor to ceiling height) to add a bit more insulation.
Slathering a layer of EDPM on the final product will serve multi-role duty of reducing noise from rain on a metal roof, opportunity for water intrusion (leaks), and indoor temperature from sun exposure.
By the way, we love RV porn, so feel free to post lots of repair-in-progress photos.