"If you're not sure what the correct tire pressures are, check the placard on the driver's side door. "
This is very poor advice. The door stickers are one size fits all and cannot take into consideration how and what you have loaded. It also assumes you have the original tires and suspension which many do not. More bad advice that I have often seen on this forum is to just inflate to the max psi shown on the sidewalls. Very few coaches need anywhere near that much pressure and overinflated tires will make your coach handle like a pig on skates.
The one and only way to properly inflate your tires is to load your coach as you would for a trip and then take a trip to the CAT scale. Take the results to the your tire manufacturers load inflation table and use those numbers.
Keep in mind that as your load varies on different trips {and it can vary a lot} you need to adjust your tire pressures accordingly {again use your tire manufacturers load inflation table}. Before every trip I roll into Discount tire and have them check and inflate/deflate to the appropriate psi. They do it for free but I always tip the guy a few bucks and thank him for the service.
:C