I tried to help with a similar issue on a 97 Beaver with Cat engine , that engine like mine ( that I found out after a muli day search ) has an in-line 20 amp fuse for battery power to the engine ECM.
If that fuse is blown you'll get your exact problem. Water proof fuse holder usually within 18" of the chassis battery positive post.
Or look for a flat 4 wire multiple connector within 18" of the ECM, this is the battery power and wiring grounds for the ECM ;( 2 power / 2 ground ) both power wires come from the same in line fuse. If you have no power at that connector , continue your search for the inline fuse holder.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.