Welcome to iRV2, there is the possibility of a bad mechanical lift pump, or a hidden fuel filter, ( strainer ) that is blocked. These filters are small and easily missed in examining the chassis.
I'll see if I can find a picture and post it.
You posted a 250 HP rating for your Cummins engine , there was over lap in ratings in the 90s , do you know if it's a 5.9, or an 8.1 ?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.