I have few customers that live on roads with 12 % grades when I crest the hill and head down it it just stays in 1st gear with engine at 500 RPM and going down hill all on brakes. Is there a way to have the engine hold some load when going downhill. Going faster that 20mph with 101 inch wide trailer on a narrow twisting road isn't a option.

This is a tough one. The Torque Converter Clutch won't engage in 1st gear, even in Tow/Haul. Therefore the 500 rpm. And an exhaust brake won't help at 500 rpm. You could try researching to see if someone has figured out a way to fool the TCM into locking the TCC in 1st gear.

You might check the temperature of your brakes at the bottom of the hill to see if they are overheated. If not, maybe you are ok. Make sure the trailer brakes are carrying their share of the load. A temporary controller adjustment might be in order.