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I'm wondering if this is what causes our terrible ride over even the smallest expansion joint. RV came with F range Michelins and I'm running G range Dunlops at 85 psi in the front (using the Goodyear tire chart)
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Probably NOT, assuming the tire size is otherwise identical. The higher load range basically just extends the inflation chart to higher load values at higher psi. Where the G overlaps the F in the chart, the inflation pressures should be the same. The slightly stiffer sidewalls make very little difference, usually imperceptible.
However, you also changed brand & tire model, so there may be differences in construction that make the Dunlops stiffer.
Modern tires don't increase the number of plies like in the old days - they just use stronger materials in the sidewalls & belts to get a higher capacity.
Radial ply tires are inherently springy and supple in the sidewalls due to the "radial" direction of the cords that form it. That's why they bulge or squat more than bias-ply construction.