Take it for what it is worth, I have had two campers in the past 10 years both five years each and so far never had a problem with snow on the roof vents. Now last year here in the North/East we had a record breaking snow fall, we had somewhere between 34" to 46", this was a collection of the white stuff from three snow storms 4 days apart and did not have any problems this Spring with leaks or cracks in the vent covers. They were not covered and there was no heaters operating in the camper. I still inspect them like other things before Winter hits and in the Spring.
The a/c unit as well is not covered, I don't want to trap mosture in the unit it will dry out faster if it is not covered.
One thing you didn't mention...do you have MAXX AIR VENT covers over top of the vents?