Our entry level diesel pusher coach came with a 10 gal water heater and I never have a problem of using all of the hot water and never turn the water off during my shower. I guess, as others said, it all depends upon what you consider "long". My showers are usually about 5 - 7 minutes.
The generator will provide 50amp power to the coach so that you can use any of the electrical appliances the coach may have. In addition, it also supplies power to the converter to keep your batteries charged so that you have all 12vdc electrical items available.
Also, as mentioned, some coaches come with a water heating device (usually runs by diesel) that will provide endless hot water as long as the water supply is available.
Hope this helps
Ron