Why doesn't this exist?
Take a small diesel like the ecoboost, pair it with a front wheel drive unit and put it in a heavy duty truck. Then put electric motors on the rear wheels. You can have regenerative braking and alternator charging for the batteries. When pulling a fifth-wheel you need torque when starting and going up hills. This is when the electric would work with the ecoboost The rest of the time the ecoboost could easily keep the heavy load moving down the road.
And when you are not towing, either the ecoboost or the batteries could be used.
Once we can get the R & D department in Washington to meet in the middle instead of all or none. There are a lot of solutions.