Because most vehicles have OEM tires that have yust enaugh loadcapacity to carry the GAWR's, and most Recreation-vehicles are loaded to their max in general use, you would expect AT-pressure given on sidewall.
This is what the Vehicle-makers base their advices on.
But , even if not overloaded on the axles, there can be misbalance R/L .
Then you need the pressure for the highest load on the axle.
This then is higher then AT-pressure given on sidewall , but this is not the maximum allowed cold pressure of a LT tire and Truck-tire.
Once determined of a ducument of Semperit/Continental, that maximum cold pressure of tire is 1.4 times the AT pressure.
And most tire makers in America still allow 10 psi above AT-pressure for LT and 20 psi above AT for Truck tires ( in fact a 22.5 inch tire on an RV is a truck tire to my opinion) . They even advice it for higher speed for ST tires.
So in the end you can better use higher pressure then AT- to give the tire the extra savety.
When you have weighed, and then preferably per wheel( pair) then you can determine the needed pressure more secure, but as long as you did not do that , you have to be on the save side.
I am able to calculate it for you , but need all the data for it.
Will give link to topic I made , so I wont polute this topic further with it.
Will Edit it , first have to find it.
Edit: found it , its about pressure -loadcapacity lists and and will use that topic for further calculations.
Another EDIT: tried the link and wanted to write a new comment, but could not because the topic was to old already.
So in time I wil open a new topic for it , but the text I copied and used for other topics , I will place here , so you can see what data I need for calculating.
Tirepressure advice is all about load on tire and speed ( and sometimes about alighnment - camber angle).
So if you can give details of car and tires , I can calculate an advice pressure with some reserve for things like, pressure-loss in time, unequall loading R/L, incidental extra load, misreadings of pressure scales,and misyudging of weight, etc.
This is from tires next and can be read from sidewall:
Maximum load or loadindex.
Kind of tire to determine the AT-pressure/pressure needed for the maximum load up to maximum speed of tire, or if lower 160km/99m/h/reference-pressure, wich is not the maximum pressure of tire.
Maximum speed of tire, most given as letter ( Q=160km/99m/h,N=140km/86m/h fi)
If you have offroad or tires looking like that , with large profile blocs that cover a part of sidewall, also mention, they are allowed lesser deflection then a normal road tire, then the tire maker used to determine the maximum load (to my conclusion the case for the Bridgestone tires on Ford Explorer in the Ford/Firestone affaire).
If you cant find all of it give sises of tire and Loadkind, then I will google for it.
From car next and mostly can be found on same plate as the original pressure advices:
GAWR and GVWR ( Gross Axle/Vehicle Weight Rating)
But best would be to determine the real weights in your use on seperate tires or estimate it as acurate as possible, by weighing per wheel(pair) or axle.
Maximum speed , you dont go over for even a minute in your use, eventually different for different situations, for instance when towing or fully loaded.This apart from trafic regulations, if you drive faster then allowed give that speed. Nature punnisches with tire-failure, police only with a penalty.
Give all that and I will calculate and give a picture of one of my filled in spreadsheets in my answer.
If other then original tires, indead as is already answered other advice is needed, a stiffer tire ( fi C-load instead of P-tire) needs a higher pressure for the same load, or the other way around, has lower loadcapacity for the same pressure.
Greatings from a Dutch pigheaded self-declared tire-pressure-specialist.