Would you ever tow a horse trailer carrying a horse, to get you out of a bind? Either trust the tech or not. Tesla battery takes 100 Kwh. A little gas genset is a drop in the bucket. Running a 2 KW genset for an hour is only going to give you 6 miles, best case. Likely be less and where are you going to store the fuel? Inside the car? Really, really bad idea. 
Guys that do serious road tripping have a much simpler solution. If the system shows you running out of juice before you get to the Supercharger, all you have to do is slow down, just a bit. Speed kills range and slowing just 5 miles an hour for half an hour will more than extend your range by a bigger number than running a dinky generator.