Tire Air-Presure Preparation: timing

as long as the trailer has been parked for a few hours it won't matter one bit when you check the pressure. Yes, it can vary slightly as ambient temps change, but really not an issue.

It only matters if the temperature is expected to be more than 20F greater or less tomorrow.

you should check your air pressure when the tires are at the current outdoor air temp and not in the sun. If your traveling and do not expect the temps to change much a day before is ok , I check mine the night before when the tires are in the shade and the tire are cool, If you have a TPMS you will see the pressure go up as the tires warm up also the sunny side will have higher pressure than the shady side as you travel. its best to have them cool like the sidewall says xx psi cold

As others have said, it doesn't make much difference in terms of pressure changes unless the temperature is changing wildly overnight.

If one (or more) of the tires are quite low, I think it would be a good idea to double-check them before leaving in case there is a slow leak. If they're all relatively close to the correct pressure and all about the same, it's a pretty good guess that none are leaking and that they won't develop a leak sitting parked overnight.

I check mine first thing in the morning before the sun start heating them up the day before I am leaving.

I asked a similar question on a different RV forum and did not get an answer. If I understand it the OP is asking about ambient temps for setting tire pressure.

If we do that first thing in the morning before starting a trip the ambient temps might be 60 degrees. If we check later in the day, we do need to make sure the vehicle has not been driven and we need to be sure the tires have not been sitting in the sun. Now the ambient temp might be 85 degrees. Setting pressure at one time versus another can have a very substantial effect. That is what happens within one day's time. Over the course a few days ambient temperature fluctuations can be considerable. I am not sure what makes sense. One thing that does not make sense to me is trying to constantly change and readjust pressures due to daily changes. If the temps are changing when we are setting pressures, they are also changing when we drive at different times of the day.

My rear tires are running at very close to maximum load capacity and I set the pressures at the maximum. I try to error on the side of higher pressure. That means on a cool morning I might be at or just less than the max pressure. On a warm day I might start out several pounds above max.

Pressure is proportional to temperature in degrees absolute (freezing point is 273, boiling 373).
An increase from 70F (294A) to 80F (300A) causes tire pressure to increase by 6/294x100 = 2%.

We know air pressure changes with temperature. The problem remains how to set the pressure at one temperature when the tire is expected to work at different temperatures. Constantly changing pressures as the temperatures change does not seem feasible. One day I might be up in the mountains where the morning temp is 40 degrees. I might drive down to a warmer location where the afternoon temp is in the 90s and the morning temp was 80.

I have decided to set my pressures for 70 degrees and leave them there. That means that at 80 degrees the pressure will start a degree or two over the maximum psi. On those cold mountain mornings at 40 degrees, the tire pressure will be about 4 degrees lower than the maximum rating.

If you have a TPMS I recommend a couple days before the trip.

Then check the pressure with the TPMS at the same time of day a couple times before the trip.

On a trailer all tires should read the same if there is no direct sun exposure.

This will find any slow leaks such as a nail that was not previously known.

Might be for 2 reasons why you want to know this.

First already given for the probable other outside temperature tomorrow then tonight.
Then you must know that advice pressure is given for 65 degr F, and between 55 and 75 the difference is nihil.
But if higher temp , for instance 100 degr F , the pressure rises cold measured that much that you cant deny it.
But at that temp , the tire needs this higher pressure to give lesser heatproduction ( because of lesser deflection) , the cooling down is also less then because of the smaller temperature difference between hottest part of rubber of tire, and outside and inside tire temp.

Second reason , I think you mean, is that loaded the tire gets higher presssure. And that is not true.
Its even so , that if you fill the tire off the ground, so with zero weight on it, to for instance 80 psi, and you get it ont the ground fully loaded, you still measure 80 psi. I once checked it with my own car , by measuring several times yacked up and on the ground, and its true.

So fill the tire to best determined advice pressure for the weight on it and speed used, calculated back to 65 degr F.
And when you check, also check the outside temperature at that check.
If between 55 and 75 degr F outside, dont bother to recalculate, but for higher then 75 degr F recalculate ( for wich I give here a list). When below 55 degr F , you can fill to advice pressure, for giving same deflection of tires, and so better fuel saving, but for savety not needed, yust look in the list I give, to see if the pressure dropp is in line with the temperature dropp.

Cold pressure is when inside tire temp= outside tire temp, that simple.

When driven the bending of rubber heats up the tire inside, so you dont measure the cold pressure. Also when sun is shining on the black tire, it heats up the tire inside, so tire inside is hotter then tire outside, and you again dont measure the cold presssure.

Here the list and use it like this.
determine best advice pressures for front and back and eventual Tag axle. Then look this up in my list at the line 65 degr F , is fatt printed. Then when adjusting also note the outside tire temp so ambiënt temp. and look in the row what pressure it has to be for that temp, and fill or adjust to that pressure.

Mostly you will find that if you check and note the ambiënt temp, that the pressure is in line with the temperature. so you dont have to adjust, so do nothing.

You can also take over , only your advice pressure for intervalls of for instance 20 degr from 65 degr F. and store that somewhere.