Is this an automatic? Does the trans have a tow haul mode? Was it engages? Do you have 4x4? Were you in low range? Do you have poor or worn out brake pads?
In many auto transmissions engaging tow/haul keeps the torque converter locked in to greatly improve engine compression braking. Obviously, if you have low range, that will further enhance engine braking. If you have an exhaust brake it is more effective at higher engine RPM so low range helps there too to get the engine RPMs up.
You may need to look into upgraded brake pads and rotors if you regularly need this kind of stopping power.
25%! Wow! that's a hill for 12,500 pounds.