Rick- You are making it too hard... Here is Michelin RV tire Guide.
https://www.michelinb2b.com/wps/b2bc...s_Brochure.pdf
In your example remember tire pressure across an axle should be the same in all tires on that axle.. Where you can't balance the load use the pressure for the tire carrying the most weight. (see pg 4)
The average weight is used when individual wheel weights cannot be determined, like using a CAT scale. Using average weight, you usually add a larger "fudge factor".
Don't interpolate between weights or pressures in the chart. go to next higher value.
For XRV 255/80 22.5, for 4500#single use chart at 4620 and 90psi. Use 90psi for both front tires.
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