The claim: Rex 84 / FEMA facility. Estimated capacity 45,000.
What it really is: First, it's located near Bowling Green, Virginia, not Fredericksburg, Virginia.
The fort, while it is an army base, is also used for year around training for all branches of the military, as well as other government agencies, thus new and upgraded facilities are commonly needed there.
While it's not actually known whether or not A.P. Hill was actually used for Rex 84 training exercises, there are multiple civilian events that occur there, most notably the Boy Scouts of America's National Jamboree, which it hosted from 1981 to 2010, and usually hosts tens of thousands of children and teenagers, and no one has ever said they saw giant prison camp there.
If there really was a FEMA camp there, then why would the government allow thousands of civilians there every four years to camp out?