why wouldnt you just plug it in?
i had a 7.3 diesel (1993) and it wouldnt start unless plugged in. i didnt check into anything else, altho i did run summer oil (its cold here in edmonton ab and -40 is not unheard of) its essential to plug any diesel in.
does it start when plugged in? if you dont use it that often, maybe just take the batteries out a let it sit. that way your not having a slow pull off of them when its not in use. just an idea anyways