Okay, I'm just going to assume some information.
Just for the sake of explanation, let us assume that the tires are Goodyear 225/70R19.5, okay?
Lets start out with a simple chart.
For single tires per axle
PSI---------------70-------75-------80------85------90-------95---------100 (There's more but let's stop there)
Weight(S)------2895-----3040----3195---3315---3450---3640(F)-----3715 (F = Load range F)
Now, let us assume that the weight on your tire is 3040 per tire on a single axle (The psi setting has to be the same for all tires on the same axle)
So, let us assume that you inflate your tires to 75 psi. I mean, after all you just came back from the CAT scale and you know what your weight is. You drive home pick up the wife, 50 pound dog. The dog sits at DW's feet. Total weight is an additional 150 pounds on the passenger tire. (You didn't think I was going to make your DW heavy, did you?) You are 150 pounds overweight on the passenger tire. Even if you had distributed the weight evenly across the axle, you would still be 75 pounds per tire overweight. If you experienced a drop in temperature or altitude, you would be additionally overweight. (Remember, this is just an example - I actually like working with the 22.5 inch tires)
So do the fudge factor:
Inflate the tires to 80 psi, and the tires are now rated for 3195 pounds. Now you have a fudge factor of 155 additional pounds per tire, or a total of 310 pounds you can put in the RV distributed across the axle. See where this is going. No? Okay, now stop at Wall-world for the night, go in, buy some groceries, supplies, whatever, and make sure you go to the bathroom scale and weigh all that stuff, because you are not overweight on the tires. Don't forget that DW and Fido are using up 150 pounds of that, so you only have a margin of 160 pounds. Don't go to Mcdonald's every night and eat hamburgers. Don't fret - more fudge coming.
Inflate the tires to 85 psi, and the tires are now rated for 3315 pounds per tire. That is 275 pounds per tire, or 550 pounds across the axle. DW and Fido can be comfortably safe and do not have to have salad every night.
Now let us say that you are running at 85 psi, and when you checked the tires at that pressure the ambient temperature was 90 degrees (I'm in Texas). You are at sea level. You head North. The temperature drops to 60 degrees. That represents a 6% decrease in pressure. 85 minus 6% equals a 5.1 psi change, and the gauge will only show a 5 psi change. So you are at 80 psi the next morning when you get up. Oops, wait a minute, you gained 2000 feet in altitude. That is a 0.96 psi increase in pressure, so your actual tire pressure reading should be 81 psi. Look at the chart. Are you still within range of the tire manufacturers recommendations? Do you need to change the tire pressure by adding more air? Nope. Smile, be on your way. If you go to the other extremes, like from 90 degrees to 120 degrees you are going to gain 5 psi which will put you at 90 psi plus one for altitude of 0.96 psi so you are at 91 psi. That particular tire in this example goes to 110 psi at a max load of 3970(G Rated tire). Are you still within the parameters. Most definitely.
You can do the math for the duals, and remember their load rating for this specific tire is 3115 per tire. That is a total axle load rating of 12, 460 pounds. Your front steer tires, single tire on each end of a single axle at 85 psi is 6630 pounds.
I cannot express this enough. Never let air out of a hot tire that has not set for several hours and come to ambient temperature. The best time to check a tire is early in the morning. If the sun is hitting the outer dual, it will normally be at a higher psi than the inside dual. If both are on the shady side, the inside dual may be a tad higher that the outside dual, because the inside dual is getting heat from the RV. After about a mile down the highway they will both catch up with each other.
When to check tire pressure:
- Before each trip
- Every morning during long trips
- Before you leave and when you return home on short trips
- Before and after storing your vehicle
- AT least once per month while the vehicle is in storage
The only way to know what the proper inflation for your tires are to weigh the vehicle, preferably at each corner, and use the manufacturer's tire inflation chart.
Happy trails. If I made a mistake in the computations, well, I'm not a tire engineer nor a math major.
Edited: Here is a link to the Goodyear Tire Inflation Loading
Once there click on the blue "Download RV Tire & Care Guide."