As I recall - without having a Goodyear chart in front of me - one of the footnotes states that Goodyear does not recommend inflating to less than 45 psi regardless of the load. This was specifically for the Marathon ST tires. Theoretically, at 50 psi (Airstream Spec for my trailer) either the "C" or "D" tires carry 2,150 lbs x 4 = 8,600 lbs. With 900 lbs on my hitch, and very little load transmitted to the trailer tires with the equalizer bars, my trailer net axle weight is right at 7,400 lbs (---checked by scale but weight slip not handy right now.) That leaves a reserve of 1,200 lbs for a possible differential in load from side to side - or due to road conditions. I'm running "D" tires since my last change (5-years back) but still inflating to 50 psi. I'll put new Marathons on ("D") if I head south this winter - otherwise, I'll wait until next spring.
2003 GMC 3500 D/A, CC, LB, 4x4 and 2000 Airstream Excella 30. WBCCI 7074