When a trailer and truck are under tow (by that I mean going down the highway at 55 mph or more) the sun shinning on the tires is basically a non factor.
I just finished a 1500 mile trip with daytime temperature at 95-105°F.
My TPMS never showed more than a 2 psi difference on same axle(cold pressure 80 psi on E rated tires) after at least 20 miles of towing.
I did see pressures in the 95-97psi range on the rear truck tires (cold pressure 80 psi on E rated tire and ambient 100°F). The rear tires on the truck were the ones closest to tire rating on my rig. 2730 load on 3042 rated tires.
When the ambient was in the 65°F range my rear truck tires were running about 90 psi.
Several times one side of the rig was in full sun.
The major factor in tire temperatures while under tow is tire friction from sidewall and tread flexing.
