Looking at the phone book under 'US Government Offices': National Guard, military recruiting offices, Social Security, USDA, Farm Service, FHA, Western Area Power Administration, Dept of the Interior, Fish and Wildlife, IRS, FAA, Natural Resources, Rural Development. Geez, plenty of them around here. Some of these offices share services with other federal entities.
Unless something were to go terribly wrong, a person would probably be OK with a weapon locked up, attached to the vehicle, and out of sight. Still would be taking a chance, possibly.
So what happens when you go to town for machinery parts or supplies with your LEGAL CCW permit, and get a call from Farm Service Agency (USDA) that you need to stop in to sign some documents? Take the weapon back to the farm and make a special trip back to town without the weapon? Find someplace around town to stash it while in the federal office?
I don't know if these federal parking lots would be prohibited or not. Would you have to check with each agency?