In North America, whenever I have seen portable light towers that use 1000w M47, 1500w M48 probe start metal halide lamps and are powered by gasoline generators, I am wondering what voltage is being outputted by their generators and whether they can output different voltages such as 120v, 208v, 240v, 277v or 480v or just one voltage.

The attached file shows what these fixtures look like.

Considering the fact that these lights are used mostly for commercial purposes, I’m guessing that the generator puts out 480V L-L and 277V L-N (3 phase system).
Although I have seen versions that put out 208V L-L and 120V L-N for 3 phase units, and 240V L-L with 120V L-N for single phase units. These units are small, usually under 10-15kW.

http://www.berjayagenerators.com/products/Generac-MLT4060K%252dSTD-%252d-6kW-Towable-Diesel-Horizontal-Mast-Light-Tower-w%7B47%7D-Kubota-Engine-%26-Manual-Winch.html  This one and others I looked at are all 120/240v

At such a high wattage I hardly see these lamps running on 120V circuits as anything more than 1, maybe 2 per circuit! It's probably OK for generators, granted. But even then, it just makes sense to use 600V ballasts (or 480V) and having much smaller currents to deal with!

I feel like running the lights on 240v would make the most sense. A lot of the generator powered light towers also have a 120/240v auxiliary power supply for running tools and having a 120/240 and a higher voltage circuit for the lights seems to add more weight. 

I feel like running the lights on 240v would make the most sense. A lot of the generator powered light towers also have a 120/240v auxiliary power supply for running tools and having a 120/240 and a higher voltage circuit for the lights seems to add more weight.

Yep. That’s the more common setup.