All tires pressures need to be set based on load inflation tables which in turn uses your scaled weight per axle. The manufacturer load inflation tables will give you cold PSI recommendations.
You lose or gain 2psi per 10 degrees. 80 to 30 is 50, that's a change of 10 psi. The load inflation table is more of a "range" of +/- 5 psi, if it says 100 psi and then you move it +5 to 105psi that will not make a difference whatsoever.
If you know you are going into a colder climate, gaining altitude (you actually gain 2-3psi for every 5,000 feet up you go if temp stays constant) or going into a cold front, add +5 or +7 more psi.
The sidewall of a Truck/Bus tire lists "minimum cold recommended pressure at the MAXIMUM LOAD". Read that again, the Truck/Bus tire does not give a recommended PSI on the side wall. ie; a Continental 315/80R22.5 HA3 LR-L has a max load carrying capacity of 9090lbs per tire in a single tire application AT 130psi and a max load carrying capacity of 8270lbs per tire in a dual application also at 130psi. In a 10 tire Coach that means (9090 x 2 =18,180) + (8270 x 8 = 66,160) = 84,340lbs max spread across all tires. I know a Class A American Coach 42 with 10 315/0R22.5's GVWR is 47,000lbs. If we could pretend all tires carried the same % of load that is 47,000/10=4,700lbs per tire. The load inflation table for the Continental 315 HA3 doesn't go lower than 5675lbs dual and 6175lbs single at 80 psi, essentially even 80 is a safe range. Run them at 95 or 100, see how they feel, do they wander, does it feel stable, does the wear pattern look even? Then adjust from there. Knowing this, why would we run the PSI at the max load rating on the sidewall? That would be 130psi and overkill.
The door placard will normally list a PSI based on a load inflation table but it's not reliable. Sometimes it's for different tires, sometimes the OEM is wrong, sometimes it's for UVWR and sometimes it's for GVWR.
The best recommendation is 1) scale your axles, 2) look up the load inflation tables per manufacturer of tire and adjust accordingly.
Load Inflation Tables
