If you wait until after the sun is up, then the readings on the sunny side will be different that those from the shaded size. In Arizona the end of March getting ready to depart for home this can have quite an effect!
I use really good tires on my trailer(RIBS and now R250's), I check the air pressure twice a year. Once at the beginning of the snowbird trip and once at the end before leaving for home. In between I shot them at fuel stops with a heat gun. In 8 years I have only found one tire low before leaving Arizona and replaced the valve core.
People report having to add air on a regular bases. Better grade tires might fix that situation. Chris
