Why diesels are most efficient around 1,800 rpm

The "most efficient gear" to pull in the transmission is whatever number "direct" 1:1 is.

My old irrelevant Cummins with a 6 spd manual will drop 100deg off the EGT shifting out of OD 6th to direct 5th at the same road speed.

Does 100deg of exhaust temp mean more or less fuel?

My 6spd auto F-350 heads for 4th (direct) asap it has to lean into the harnesses'z.

Neither of the above have 1000 claimed lbs ft of torques or 500 horsepowers. They need to rely on their gearing once in a while.