Diesel Bombers - 2003 Excursion 6.0 no start

I could really use some help. We have a 2003 Excursion with 6.0 that will not start. It all started about a week ago. We got fuel, about 15 gallons, on Wednesday of last week. We drove it home, about 50 miles, parked it and plugged in the heater. We got up the next day and it would not start. I tried several times until the battery was almost dead. It would not start for 3 days straight. I was trying to start, but just would not turn fire up. It was acting like a fuel problem, like bad fuel. I kept it plugged up the whole time. I tried again on Sunday and it started up after 2 or 3 tries. Sunday was in the low 40s, but it had been cooler 20s or 30s over the days that it would not start. At this point I was assuming water in fuel. I put some Howe's fuel treatment in it Sunday evening and drove it around for about 10 or 15 miles before parking it. After the treatment, it ran good. It stopped cutting out going up hills at 55 or 60 mph and had good power. We drove it Mon thru Wed just fine. Again plugged in the whole time. Took it to work on Wed, a cold day, low teens. Parked it at work, without plugging it up. Tried 2 1/2 hours later to start, but no good. Started acting the same, like not getting fuel. Today is Friday and still can not get it to start. Replaced both fuel filters, which were dirty. Someone stripped out water separator screw, so unable to check for water, but still nothing. I have tried for several hours to start it. Even placed a hair dryer on water separator unit to warm up in case of moisture frozen in it, but nothing. I am getting fuel in filter bowl on engine when tun key on. I checked codes and no codes. While trying to start it at work, I had it plugged in. It has been in 20s these last few days. tomorrow is supposed to be in 40s. Hoping that may help. I have no idea where to go from here, without throwing a bunch of parts or money at it. Any ideas would be appreciated. Sorry so long. Wanted to give all info.