Im in Melb (save the jokes!!), and I briefly used a 4 ply Premier style breathable cover, and although our summer is short it can get pretty hot and dry. After removing the the cover to use the van on one occasion, I notice a couple of bubbles on the roof, one large one the size of a dinner plate, like the fibreglass was de-laminating. With the cover left off, I checked a day or so later and the bubbles were gone ... I assumed they were caused by the heat build up under the caravan cover. The heat the build up under the cover on a warm sunny day was extraordinary.
The bubbles reappeared once more, so I never put the cover back on after that; and the bubbled roof never returned. Our 2009 16.49.1OB sat outside for 5 years from new to when we sold it a few months back, and with regular washing and polishing look pretty much as it did new.