120 PSI is right, but DO NOT EXCEED
I have a Thor Venetian, 41 ft long, its sticker says to inflate all six tires are to 120PSI when cold. I checked the tires, and they say 120 PSI MAX (also when cold). I got mine up to somewhere between 116 and 119 PSI. You don't want to go over the tire maker's maximum inflation. Lower inflation will cause more sidewall flex, shortening tire life. It will also hurt fuel mileage.
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