I'm betting you've got a Coleman Mach AC unit that was probably built around 2011 (Give or take a year.) After we bought our 2011 Tour, we'd get a mini ticker-tape parade of aluminum foil blowing out of the AC vents whenever we ran the AC. I still get some shards of foil every once in awhile, but they're not nearly as big or as numerous as they used to be.
I was told by another RVer who was experiencing the same issue, and who contacted Coleman about it, that Coleman had insulated some part of the AC unit with aluminum foil, but the bonding material they used to hold it in place didn't hold up well. As the foil came loose, it would get chewed up by the fan in the AC unit, and shot out through the ducted roof vents. I was also told that Coleman said there was nothing to worry about, and it would eventually stop. Like I said, I still get some once in awhile, but I've been running our three roof AC units for years now, with no problems.
Kev
By the way, welcome to the RVForum Barb025!)