Many people use the values on the sticker.
To get more precise you can weigh your RV and then use the loading charts given by the tire manufacturer (
Here is Michelin's). Typically these charts are where people have derived the lower pressure (I believe I'm also running 70 front, 65 in rear--its hibernating for the winter and I won't be checking for another month or so).
The rears are usually less because of the duallys.
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