I like the idea of a Darkest America thing as opposed to a colonial Darkest Africa, with (barely) surviving, second rate Euro and South American troops coming in to salvage goods and finding resistance. Maybe with New Orleans, Corpus Christi and the Brooklyn Navy Yards the last half-functioning ports as points of entry.
Twilight:2000 has a bit of that. Mexico, with help from the Soviet "Division Cuba", invades Texas, and Soviets invade Alaska. The US splits into 2 factions, MILGOV and CIVGOV.
In Texas and Oklahoma, the remnants of 49th Armored Division, with a few tanks, lots of "Hillbilly Armor" trucks, and cadets from the Air Force Academy, are loyal to MILGOV and doing their best.