You are correct when you say that XT22s and Model 5s are very common in Iowa. They're probably the most numerous mechanical sirens I've seen around here, especially in the smaller towns. As for Thunderbolts, I've seen only one 1003 so far (Parkersburg), but the 1000/1000T is fairly common. Perhaps the 1003 is more common in western Iowa (I live in eastern Iowa, near Cedar Rapids).
As already mentioned though, there are other models. My hometown of Vinton has three Whelen WPS-2807s, with a fourth Whelen coming soon (Sadly to replace a Federal STH-10). The DAEC nuclear plant has a whole system of 144 Whelen sirens, mostly 3016s and 4004s, with a few 2016s included here and there. I personally have not seen it, but there's a rare Whelen 4008 in the Davenport area. I've seen quite a few ASC T-128s, especially in northeastern Iowa. There's an ACA Allertor in Mason City I remember seeing one day, and I got video of a dual-tone ACA Screamer in Atkins on my YouTube account (
http://www.youtube.com/profile_videos?user=bigloudnoise).
So, there are many other models of sirens here. But yes, a majority of mechanical sirens in Iowa are Federal.