first adding extra nuetral plates is a good idea. Its just a more efficient way to produce HHO. Less heat mean less current wasted means less altenator usage means more MPG.
Second why dont you just put a relay in on a switched source. Ther are plenty of them and relays are super easy to work with!!!
Third the reason your PWM is staying on is either because its limiter is set lower than 13.8 or because the battery is capable of holding that high enough voltage for about a min. New batteries are quite good and capable of good capacidence<---lets pretend thats a real word
so just install a relay already! truck turns on, so does HHO, truck turns off, so does HHO. problem solved