Automatics

And it will only go to neutral when the required torque to maintain speed is near zero.

If positive torque is required to maintain speed (without torque from the engine the truck will slow down) it will stay in gear, and engine will supply the torque.
If negative torque is required (engine braking, either with or without jakes on) it will stay in gear as well.

If the zero torque condition exist (for example small gradient downhill, gravity compensates for rolling resistance and aerodynamic drag ) it will go to neutral.
As soon as the truck picks up speed or the truck looses speed, more than 1-2 mph, (because gradient getting steeper or levelling off) the transmission will engage the matching gear again. The engine will provide engine braking or apply torque in order to maintain the set speed.

It is a weird sensation at first, and one has to get used to it, but it only works in a very narrow speed range, and in very specific conditions.
As soon as these conditions change the transmission will engage gear again.

It can save some fuel.
If zero torque from the engine is required, the engine uses less fuel running at idle speed (and zero torque at the flywheel) than when running at 1400, or whatever rpm matches the vehicle speed, without producing any torque.