The surge brakes will be the problem- they are probably constantly engaging down that hill.
You gotta remember that the brakes are what's slowing you down. Any pressure off the trailer will be transfered to the truck brakes (not good).
I'm wondering why only one wheel gets hot- the other one may not be working properly.
I have seen people that tow on hills alot add another set of brakes to the other axle- it will spread out the load a little. However, I'd make sure both of your current discs are working properly before going that route.