First, get the coach weighed with your normal camping gear aboard. A four corner weight is ideal, but axle weights are better than nothing. Then go to the TIRE manufacturer's website and look up what the pressure should be for each tire position. If you got axle weights, divide that in half for each corner, If you have rear duallies, use the dual pressure for each tire.
Most Pilot/Flying J truck stops have a scale. They charge $10 and there's usually enough space to run through the scale twice to get the weight of all 4 corners.
Max sidewall pressure is good until you do the above. Just be aware that the coach will ride like Fred Flintstone's car.
Congrats one the new coach!