What chassis and drive train?
Generally speaking, manually shift the transmission to maintain speed at the 2500 RPM number you were told. Going down hills the general truckers rule is go down in the same geat you used going up. DO NOT ride the brakes going down. If you do you will find yourself with no brakes. Instead, brake hard to bring yourself to 10-15 MPH below your desired speed, get off the brakes and let it coast using engine compression braking to help hold speed. Once you get above the posted speed, brake hard again. Repeat as necessary.