I have reflective material under my mattresses year round on both bunks and have never noticed a moisture problem above or below the reflective material. I do seasonal checks looking for signs of moisture damage throughout and have noted no changes in the OSB below. That isn't saying we don't have moisture build up, but not that I can feel personally nor see in the bed components.
A lot mileage varies here but I've also never had any condensation build-up overall inside either one of our PUPs, and I don't open anything for ventilation. In fact, I've made efforts to seal up the holes and crevices especially for the cooler weather camping we like to do. But moisture is obviously getting out somehow. I know others have had to open windows quite commonly.
I use High Wind PUGs on the bunk ends, reflective material under each mattresses and the car windshield shades in the windows (mostly to darken the sleeping areas for DS who wakes up at the slightest sign of light) depending on hot or cold weather conditions. For winter camping, I use one to two electric heaters on separate circuits that can maintain an inside temperature of 65 with outside temps in the 30s. I do keep the onboard furnace turned on but the thermostat is set to 50 just in case the electric heaters can't keep up. I've never had the furnace need to do that in this configuration. I'd rather be using the electricity I paid for as part of the site than the LP I have to refill as needed.