No benefit with nitrogen in road tires. It's a greater benefit to aircraft and race cars, though, so if you're going to run your trailer at 50,000 feet then land it on the road at 300 mph, then by all means.
My tires from Costco also have nitrogen in them, and every time I air up to maximum pressure for towing, I just use whatever comes out of my air compressor, thereby ruining the 100% nitrogen fill. I've never noticed any difference between nitrogen and regular air.
Atmospheric air is 78% nitrogen anyway, with 20% oxygen and the rest is carbon dioxide and trace gasses, so that's good enough. The "experts" say that the benefits of running with nitrogen are that it's a drier gas, it doesn't lose pressure in cold temperatures, and a bunch of other stuff. Again, I've never noticed any difference.
And I'm definitely not going to pay the $5 per tire some places charge!