Isx 15 will not start

[CumminsFreak]

Hello, I have a following issue on my 2012 Kenworth t660 with Cummins Isx 15 cm2250 engine. I left sleeper fan on one time over the weekend, came back and noticed the fans, started the truck to see if it completely drained my batteries, it did start but was running weird, low rpm and then it just died and would not restart. I replaced all the batteries, still would not start. Pulled out the fuel pump which I know often caused problems, it already has been upgraded to steel plungers but one of the plungers was stuck.. replaced the plungers and rollers, and still no start. On insite it shows me that it wont hold the fuel pressure in the injectors. We replaced the shut off valve thing located to the front of injectors on the rail and still nothing. Any advice on what should we look for next? Thanks in advance!