for the going up the hills drop it into 3 rd gear, forget even trying to keep in O/D.
for going down steep inclines, what truckers do is get to the speed limit, then hit the brakes till you've slowed 10 mph, then back off the brakes, let it coast back at its own speed back to the speed limit, repeat as needed. dont ride the brakes all the way down the hill, your brakes will melt. the using and then not using the brakes lets then cool off, if they never cool they glaze up, and dont work. then you need new brakes.
leave the trans in 3 rd gear too so theres some drag off the engine. dropping to 2 nd is not a good idea, the trans wasn't built to slow you down.