I used #8 - 1" long flat head zinc plated steel screws. I like the strength of the steel vs brass or sst for this application. If I see any sign of rust on the screw heads, I will apply white paint. I have been on the road for three weeks with lots of rain and no rust yet. Also, look around the RV and see how many other screws are zinc plated steel with no problems. I don't think silicone sealer will be necessary as the existing sealing material under the flange squeezes out under the flat head screw. My 2010 had countersunk holes, if you have drill the countersink then you might need some sealer.
Bob