The first think I do is install Maxxair vents on all the roof vents.
We leave the Fantastic Fan (w/thermostat) running during the summer, without any issues of water getting in. The other vents stay open all sumer long.
We also leave the vents open all Winter long and we are in Canada, and we have never had any issues of snow getting in.
2009 Salem PT 403FB
2005 Ford XLT Supercrew