The general method for arriving at good tire pressure involves getting the weight of your rig. Preferably, for motorhomes, the weight at all 4 corners. If you cannot get the individual 4 corner weights, at least get the weight of the front-end, then the back-end, and finally the entire coach. When you weigh it, you should have everything loaded that you are normally going to take, including fuel, water, propane, clothes, dishes, food, etc.
Then, after having these weight figures, you can go to the tire manufacturer's web site. Most of the manufacturers put tables on the web that give you the weight vs tire pressure for all their tires. So comparing your actual weight figures to the charts will give you the ideal tire pressure.
The next step is to consult your weight ratings of the coach. You should be able to find the total weight rating of your axles. Obviously, your weight figures should be less than the axle ratings. If not, you are either going to remove some items or rearrange them.
At this point, you know what is the minimum tire pressure you need to run at. From there, you can adjust the pressure up to the maximum pressures as stated on your factory sheet. I always carry about 5 pounds more pressure than my calculations showed to allow for error and last minute items added to my load (forgot the beer?).
There are many places where you can get your coach weighed. I use a moving and storage company near my house that has a scale. A lot of truck stops have weigh stations so the OTR drivers can be sure their rig is under weight.
Out West Somewhere
2016 Sunseeker 3050DS towing 2006 Hummer H3