Hi Slownsy,
A 400 watt inverter running flat out draws about 40 amps @ 12 volts. If it will be used several hours per day, I'd maximize the solar install if planning to not run a generator.
Really an energy audit would be best. Even better would be actual amp-hours used. Perhaps the hardest thing about setting up a solar system is deciding on the number of watts.
Mine was just fine when I was part time. Now that I'm full time, I wish I had 1000 watts. The only mistake I made was not allowing for expansion of the number of watts.
I can say that nearly everyone loves what solar does for their camping experience.