These look like MaxxAire covers.
As SHO mentioned, they don't mount to the roof, they have small "L" brackets that you mount the "L" bracket to the vent sides, then a bolt holds the cover to the bracket. The bottom of the covers will actually be about 1/2 to 1 inch above the roof.
IMHO - These are an absolute must for any MH or TT. They protect the very fragile vent lids, ,and you can leave the roof vents open without worry about rain or the wind ripping them off.
ALSO - Raccoons haven 't figured out how to get in them, They do know how to open the roof vents and get inside your MH
1999 Fleetwood Southwind 35S (Ford F53 6.8L V10) - Toad 2003 Saturn Vue.
It won't do MACH 2, but I can get a sandwich and take a pee.