Since the Fremont factory is out of room & completely occupied with the Model 3, my best bet would be
their European factory. Believe me, I heard about it only recently, but Tesla appears to have a re-assembly plant in Netherlands capable of 450 cars a week. It's a way to evade European import tax. It's only 11 acres, or about 0.47 million square ft., which is a tenth of the Fremont location. Not to mention, the Semi will be rather well-received in Europe, much like EVs.
Tesla did apply for an expansion permit which has been now granted by the Fremont City Council, effectively doubling the available area, adding 4.6 million sq. feet.
They've already announced plans for China & Europe gigafactory, but best believe they're now postponed along with everything else as Tesla favours all attention towards negating Model 3 delays.
I don't think the Nevada Gigafactory would be viable location for vehicle manufacturing, and it Tesla does decide to manufacture there, it'd be rather short-lived & temporary likely due to not having any other alternatives. They're already having trouble finding suitable employees skilled at manufacturing batteries let alone manufacturing cars.