My surmise is that they (the unused ones) are in staging areas along the Gulf/Florida coasts. And rightfully so. Hurricanes hitting the eastern seaboard are very rare by comparison. It'll take a few weeks to relocate them northwards.
The "used" ones were auctioned off a few years back.
The FEMA trailers were given away, at least as many as they could find takers for.
The takers had to be public/non-profit entities that could meet a silly standard (as if it were better for the trailers to go to Big Bird than to let an ice fisherman have one).
My little hometown got one for their marina and also for their little one-strip ‘airport’. I think they had to drive from North Dakota to pick ‘em up.