I had a problem with my Onan 7500 not starting. It turned over, and sometimes tried to start. I got a code 36, which says it is the fuel filter, or the air filter, and something else. I replaced the fuel filter, checked the air filter, etc, etc. Finally I pulled the diesel line that goes into the fuel pump, put my finger over the fuel pump stub, and barely got any suction when my wife pushed the prime button down. I had to use my dremel to ream out the hole to get the bolts holding the fuel pump in, but, after replacing the fuel pump, everything works fine. Make sure you really tighten the fuel line clamps.
2002 HR Endeavor