You should be fine with the rig as it is. Our Carriage Carrilite scale weighs 12,840 including he pin weight loaded to go and the truck is Chevy D/A CC LB 4X4 and usually has our 106 gal aux tank in the bed. Almost the identical weight as yours. We have zero issues with the OEM engine brake and seldom ever have to use the truck brakes unless the decent is more than 6+% and long. You also can manually over apply the trailer brakes occasionally to slow the rig down as it pulls backward on the truck and eliminates any fishtailing or sway possibilities. I occasionally use the trailer brakes pushed manually just a little bid harder than it normally is along with the engine brake on steep decents before going into a sharp or sharper curve. Eliminates any push effort from the 5th wheel behind going into the curve.
Be sure to tap the brake pedal once before starting down a long grade so the tranny is in a low enough gear to be effective before your rig has already gained lots of speed. Much harder to slow a speeding rig than to maintain the slower speed starting down from the top. You'll learn fast.
