I want to pass along a real find for 8.1 owners. I purchase ours two months ago. It had 13,500 miles on it. It was very hard to start when hot. I would have to hold gas pedal down and then it would start. It idled really rough and exhaust fumes were bad. I had replaced MAF and MAP. I looked for vacuum leaks. Not helped. I googled many RV sites with suggested but none helped. I googled 8.1 hard starting and found suggestion form mechanics. He was very sure it was fuel pressure regulator. The fuel pressure regulator is on injector fuel rail on drivers side top rear of engine. It has a vacuum line. The test was to remove vacuum line and then turn ignition key until lights on dash come on, this is also when fuel pump gets power. With vacuum line off and key on no fuel should be present. If any fuel comes out fuel pressure regulator is bad. I replace mine. I purchased one for 2002 Chevy 3500 pickup with 8.1. Install and instant smooth idle and instant start up. Really easy fix. The fuel pressure regulator is held in with inside snap ring.
I also pick up 1.5 to 2 mpg. This 8.1 is now a beast.