Like Jim says, 12 gallons is plenty. Sheri has very long, thick hair and has no problem. When we are in a hurry, we can both shower with no waiting taking 'navy' showers. We also use that method when dry camping to save water. You get wet then shut off the water, ( there is a handy button on most shower heads ) soap up and scrub with no water running, the turn the water back on and rinse. We have even had 4 people shower like this with no waiting.
As to the plugs, no problem with strips or extension cords as long as you dont overload the circuit. Your breaker panel should protect you like at home. Dont forget, you only have 30 or 50 amps at hook-up, or whatever your genny puts out. Not hard to exceed your power source if you treat it like your house. Common sense and moderation goes a long way, with electricity and water usage.