it's not a 'single' breaker, but actually TWO individual 30amp breakers, though only ONE trip handle, shown as '30amp'....which means up to 60amps at 120v between the two.
With a 58amp 7000kw Onan generator, it makes perfect sense.
basically, up to 29amps goes to one line, and upt to 29amps to the other, both fed back to the different two 'sides' of your Main Panel.
Generators that don't output 240v are used since your RV doesn't NEED 240v power, only 120v to your Main Panel. Now, while you might argue that then 'why' do you need to plug into a 50amp 240v campground outlet - it's because the electrical grid is designed differently than a generator. With the grid, you won't find a 100amp 120v outlet, so a more 'normal' 50amp 240v outlet is the 'common' campground or rv park outlet for larger RVs. A single 120v 100amp sized wire would be SO THICK that it would make your shore cord unmanageable.
The same idea surrounds the generator's output with TWO breakers - it reduces the size of the wires needed that lead back to your Main Panel, which sometimes can be at opposite ends of the coach, especially in diesel models. Even though it might seem odd that the generator's breaker only shows 'one' breaker handle, it actually is not that uncommon, but the fact that you don't 'see' that other breaker might lead one to believe that the generator only produces the power of that single breaker.
'14 Thor Palazzo 33.3 compact diesel
KingTailgater2 Dish, 100w/5amps roof SOLAR, BlueOx, 100,000+mi since '14... US, Alaska, and Canada... GA, NC, now N AL : )