Changing out a gear set could , marginally improve fuel economy , but the increase could be so slight that verses the cost of the gears it might never be repaid in fuel savings .
The drop in acceleration rate, downshifting to climb hills instead of holding a higher gear.
The 2 major factors in diesel MH fuel economy are driver habits and route selection. A light foot and staying on major highways to maintain a reasonable cruising speed will get the best mileage.
Any savings from the slight ratio change your suggesting could be accomplished by slowing cruising speed by 3>5 mph , achieving the same cruise RPM drop.