Most people adjust the air pressure to level up the truck with it loaded and the 5th wheel hitched up. At this setting, the suspension and pinbox on the 5th wheel should be adjusted to ensure that the 5th wheel is level as well and that you have at least 6" clearance between the 5th wheel and your pickup bed rails. So, whatever air pressure it takes to level the truck would be what you're looking for.
2016 Ram Longhorn 3500 Dually 4x4 CCLB, 385/900 Cummins, Aisin AS69RC, 4.10
2014.5 DRV Mobile Suites 38RSSA #6972
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