A surefire way to eliminate the sun factor is to travel when the sun isn't shining. After making sure the tire pressures are adequate and roughly the same, get on the road before sunrise one morning, travel on the highway for 30-60 min, and then feel both the tires and hubs. If one side is still significantly hotter than the other, you know you have a hardware issue. Bad bearings, bad tires, riding brakes, significantly overweight trailer, etc.
One thing is for sure -- you'll enjoy a nice sunrise.
I promise you things will get much easier once you get some of this stuff smoothed out and become comfortable with knowing when things are great and when you need to stop and address something.
Dave