Different "trusted" battery - sadly - same results.
It starts fine with "fan on "only but will not with fan and heat activated. I realize that the toaster grids won't turn on without the fan just saying it's the toaster grids that are causing the grief. And like I say - once the truck starts you can fire up the heat and it works just fine. Have a brand new heater assembly sitting here - techs may install it and see if problem goes away. That will at least fix it or eliminate the heater itself as the culprit.