Odd issue with Accelerator Pedal

Looking for help from someone regarding a 1997 pierce pumper fire truck which won’t accelerate. Purchased for use on construction site and can’t get to accelerate. Truck has a Detroit 6V-92 engine and starts just fine. Issue is when in neutral, I can press accelerator and truck revs to 1000 rpm but won’t go higher. When in drive, won’t rev at all and idles at 600 rpm. It seems like there is a Gove or setting that is causing the engine to not rev. It is in “drive” mode, not pump so I don’t think it has to do with issues on the trucks pump system. It has an electronic gas pedal and I haven’t been able to figure out how to rev engine manually. Any help would be appreciated