I mounted an XM antenna and brought the lead in through a roof vent. The wire is small, and I drilled a hole through the metal flange that sticks up that the lid seals against. I sealed the hole with a wad of butyl putty. Since the hole is above where any water puddle on the roof might rise to, and since I have a cover over the vent, there is no leak.
The antenna itself is mounted to a small piece of sheet metal, which is also attached to the metal flange of the roof vent.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch