Truck Repair: 6bt with no power, motorhome chassis, injector pump

Question
I have a 94 6bt in an oshkosh motorhome chassis.  It runs great but has no power at all sometimes it wont run 40mph on level ground sometimes a little better. When it wont pull it has no smoke at all if it runs ok it has a little black smoke. Several people have said its the injector pump but it runs great out of gear. It is an inline pump.  

Answer
I would check the shutdown solenoid first,then get the fuel pressure checked at the pump.It should have around 20 PSI with the manual operated plunger(primer pump plunger).If it is low try a overflow(return line check valve)valve on the frt inside of the pump.
If the shutdown valve is the small one it probably is worn where it has a pivot in it.Bump the starter or crank it up and see if you can move further upwards,if so it is bad.They sell a kit for around $250 that has new solenoid, brackets,& bolts to install it.