hanla05- while weighing the axles is better than nothing, the more precise way is to weigh each wheel on a tire platform scale. Vendors do this as a service (yes, they charge like $15/wheel) at a lot of the larger RV rallies & conventions. Weigh the RV with full water & fuel & equipment load. This will give you your heaviest wheel load- add 10% as a safety factor & then look up the recommended inflation pressures on the tire manufactures tire chart for your make & tire size- it will show single & dual columns for respective inflation pressures.
On my present coach I carry 100 PSI in the front tires- Michelin XZA 365/70 R X 22.5 LR L & 90 PSI in the rear duals & tag- Michelin XZA 315/70 R X 22.5 LR L- those are some really big heavy - 24 ply tires!