HOW to hook up the noisemaker with US current , one big 120V output or two 120V legs has always been as concern,
Here is one mans opinion, as lifted from the busnuts.
Halving the voltage doubles the current, which means you'll need bigger breakers, wires, switches, etc., as I wrote.
Basically, on a coach, generators smaller than 4kW should always be strapped for 120v, and generators 12kW and larger should always be strapped for 240/120 split phase.
Between those numbers, generators between 4 and 12 kW, which voltage you choose can depend on a number of factors, one of which will be the cost and availability of parts.
Take, for example, a 10kW generator set, a rather common size. 10kW is about 42 amps at 240 volts, but it's a whopping 84 amps at 120 volts. Because breakers don't come in those sizes, the required generator output breaker would be a 40-amp two-pole for a 240-volt set, or an 80-amp single-pole for a 120-volt set.
With the 40-amp 240-volt setup, you would have the option of wiring that directly (through the 40-amp breaker) to a NEMA 14-50R receptacle into which you could just plug your shore cord, a simple and inexpensive way to switch between shore and generator inputs. With the 80-amp setup, you could not use this method, and would need a transfer switch.
Any transfer switch you get for this application would also need to be rated for 80 amps, but as you are probably aware, most such devices made for the RV market are 50 amps (or worse, 30 amps). An 80-amp switch is an industrial item, much larger and more expensive than the commonly available 50-amp models.
Then there is the matter of wire sizing. The 40-amp setup would require only #8 wire (in THHN, for most normal temperatures). The 80-amp setup would require #4. If you've ever tried to pull three #4's through conduit, you'll know what a bear that can be. Also, you'll need much larger conduit for three #4 than you will for four #8.
Lastly, the rig will still need to have a split-phase entrance, because shore power at those ratings is 240 volts. So now you also have to have a way to connect your split-phase shore power to this system designed for 80-amps single-phase. Several ways to do that, but it is an additional layer of complexity.
By contrast, a 6kW generator produces only 50 amps wired for 120. So if you already have a 50-amp split-phase electrical system, whether with a transfer switch or with a cord-and-plug, it is a simple matter to just gang the two legs of the input together between the transfer mechanism and the generator breaker.
For these reasons, if it were me, I would strap for straight 120 up to 6kW, and for 240 above 10kW. In between, you need to do the math, and figure out what your priorities are.