A few years ago we had an F350 which we used to tow a Heartland 5th wheel. We were at a rally and had the rig weighed per wheel. They were able to tell us per wheel what the proper inflation should be. The proper inflation does depend on the weight. The more weight the higher the inflation as long as you don't go over the maximum. Check out the information on this link for more information.
http://m.discounttire.com/dtcs/infoAirPressure.do
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