My experience has been (as with most things TSA) that it absolutely depends on which airport you're at and perhaps which screener you get. For me, most screeners have been ok with just pulling out the camera bag from my larger bag. I will note, however, that since the shift to the newer CT scanners, I find they semi-regularly need to take a second look when I pack larger/denser lenses anyway.

I won't say that theft doesn't happen at checkpoints, but if you're halfway paying attention to your belongings, you shouldn't have any issues. The CT scanners are slow enough that I'm usually waiting for my bags and watching as they emerge. No one should be able to wander off with a full-frame camera without you noticing.

Have fun in the United States. I hope you're able to capture some interesting photos while you're here.