I think the issue is not enough horsepower at the rpm you are at. While gears multiply torque, they do not multiply horsepower.
According to the math, it takes roughly 200 hp to tow a 10k trailer with a 7k truck up a slight 2% grade at 70 mph. It takes 230 hp up a 3% grade and 270 hp up a 4% grade. From the dynos I have seen, I don't think the 6.4L makes that much horsepower at that low rpm. In fact, it drops pretty considerably from the 410 peak hp at 5,600 rpm to 327 hp at 1,600 rpm less where it is making peak torque at 4,000 rpm (429 lb-ft X 4,000 rpm / 5252 = 327 hp). Dropping down another 1,500 rpm to 2,500 rpm, and the truck might not be making enough horsepower for even a slight 2% grade at 70 mph.
There is also the factor that these horsepower numbers are at sea level with near-perfect humidity and temp. Most will be making 10-20 hp less in the real world. Even less if you live at high elevations or in extreme weather.
In contrast, the diesel you bought is making about 310 hp at just 1,800 rpm due to the massive amount of torque the engine is making at that rpm. More than enough horsepower for even 4% grades at that speed.