"I am looking for a formula for adjusting "cold inflation" pressures for large tires. I have read that for smaller (car) tires the recommendation is 1 psi per 10 degrees F.
What is also not clear is what the assumed "cold" temperature is. Is assumed to be 65 F, for example. So if we are sitting at 25 F, what is the adjustment?"
Here is my blog post that explains "Cold Inflation"
We are not in High school chem class so there is no adjustment need to get to "standard " temperature.
If you are running a TPMS, which you should be, you will soon recognize the normal range of temperatures and pressure variation in your tires.
I would not worry about adjusting inflation pressure is today's expected temperature is +/- 10F from yesterday. After all, you should be running +10% over the inflation you need to support the measured load.
Let's say your minimum needed is 90 psi. Adding 10% that means your cold inflation (cold meaning the tire was not driven on or in direct sunlight for the previous two hours) should be 99 psi so you round to 100 psi. All is good.
The next day's weather is 20F colder. That would mean the tire inflation will drop 4% ( 2% per 10F change)
So now your tires would be 96 psi but since you have to stay above 90 I would say there is no reason to worry about adding air. Just enjoy life and head out.
If the next day your weather went up 10F instead of down that means tour "cold" inflation would go up 2% from the 96 or to about 98psi. Again you don't need to do anything.
I would not worry about lowering the pressure until I saw pressures 10 psi above the pressure molded on the tire sidewall. When you get to that point you could bleed off a few psi but remember if the weather gets cold over the next couple of days you will be needing to add air again.
Don't get your shorts in a bunch about inflation. Just watch your TPMS and you will learn what is normal for your coach.