I’ve asked before about the viability of rendering corpses to make Diesel fuel. I don’t recall how to do it, but I recall it can be done. And I’m sure there are warehouses around that are full of fertiliser. If someone were to build a zombie processing plant to make Diesel, then there would be fuel to run generators and trucks (and possibly turbine-powered aircraft). It would be a good business. But then someone comes along and says, ‘Here’s the deal: You give me half your shit.’ ISTM that someone could happily supply fuel to the usurper for a while. And then one day, load up a few tankers with zombie-Diesel/fertiliser mix (What’s the ratio, anyway?) and drive them right into the usurper’s camp. I don’t know how the zombie bombs would be detonated. They’d have to explode within the camp, but before the ‘fuel’ could be offloaded. Unless you can find a suicide bomber.
Of course a smart Evil Overlord would have his assets dispersed, but a ‘hands-on’ Evil Overlord might want to stand on top of the tankers/trucks loaded with barrels to gloat.