The only way. THE ONLY WAY to determine the proper tire pressure is with a scale.
First: you go to Mitchlin and download the inflation chart for your tires
THEN you scale it. ALL 4 Corners.. (Remember there are two charts one for duals one for singles) You can find instructions for weighing elsewhere on teh forum or I can type 'em out for you.. Or you can contact www.rvsafety.com and follow the links there and they come out.. Charts in hand.
From the load placed on the tire. you determine the inflation.. Mine is 115PSI same tire.