Truck stalling out when coming to a stop.

Hello, it's going to be wild guessing until you perform hands on testing. At first glance, I might say that lack of fuel could be causing the stall and no restart. But you've mentioned poor fuel mileage, so I'm wondering if it's more too much air coming in and your fuel keeps dumping and can't keep up. Seek guidance at a shop near you; this is not a difficult diagnosis for someone with experience, tools and reference materials. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops