Recent truck engines are able to reach 50% BTE through a lot of attention to detail. There is no silver bullet, just many small incremental improvements to things like low friction bore coatings, thermal barrier coatings, low leakage injection systems, Miller/Atkinson valve timings, modifications that enable a reduced demand for lube oil flow and pressure such as controlling piston cooling flow to exactly match piston thermal loading instead of continuous full flow, variable displacement oil pumps, electric water pumps, further refinements to the combustion and boosting systems, down speeding, etc.
Look for the DOE SuperTruck presentations from the Annual Merit Reviews for more detail.