When I painted the skirts on our Starflyte, the first time, which was in 2012, had no issues with the harsh winter conditions of the NC mountains in Ashe county. Once we moved back to Charlotte, had to make repairs to the skirts and the paint was still intact on the ss screws.
So, in my case, it worked for me.
As for McMaster Carr, always bought my screws in bulk from them and usually had them in hand in 2 days, since they were coming from Atlanta.
I usually buy my screws from the local hardware store if zinc. They are about the same money as ordering and shipping.
For my fancy stainless truss head screws (think canned ham with real aircraft aluminum), I order from Fastenal. Believe it or not, they charge shipping, even when ordered from the local store. Screws aren't cheap.
All of the screws I used on my trailer were Stainless Steel. I have had no issues whatsoever after 5 years. Theoretically, zinc and aluminum aren't as compatible as SS and Aluminum, but I doubt you'll ever see any issues either way. I'd be concerned with rust if using zinc, but if painting it'll be fine to.
I ordered most of my screws from Boltdepot.com. Cost for stainless steel online is similar to the cost of zinc screws at the local hardware store.