Homemade APU question.

Hey Guys! I am absolutely no where near owning my own truck. However if I did. I was wondering If I could build my own auxiliary to the currently available EPUs or Electric APUs. I was thinking of something as simple as a small diesel engine probably a nice 3 cylinder Kohler and a Truck alternator. I would want to wire it in to fire up to recharge my battery while parked instead of idling the engine to recharge when the voltage gets low. I was thinking that this hybrid system would get the best of both worlds. I wouldn't have to run an APU full time resulting in reduced fuel consumption. I wouldn't have to carry gasoline as I have seen in other improvised APU builds. I would be CARB compliant. I wouldn't have to worry about anti idling laws, or having an engine making noise all night long. (wouldn't bother me, but it would be nice for my neighbors in the lot.) Any thoughts, or is this system already available? (I would still really want to build my own. Don't want to waste those years in Tech School.)