I purchased a 2003 Holiday Rambler Vacationer with a W22 Workhorse chassis and Chevy 8.1 engine. I have always had Ford 460 or V10 engines, so I am not real familiar with this one, so I seek your adv...
Have you run fresh fuel though it? A load of premium with a can of Stabil may do wonders for it. Also, Chevron with its "Techroline" does a good job of cleaning fuel systems. You might run a few load of that through it.