My motorhome was a 2000 Itaska and these were the service manuals I bought for it.
I looked in the front pages and it doesn't give a year range for coverage. The motor volume covers everything from a 4.3 L through a 7.4L engine.
I found this
Workhorse Custom Chassis began in 1999. As Chevrolet decided to get out of the RV chassis business, Workhorse took over those operations, beginning with converting the Chevy P30 Step Van/RV chassis into a similar Workhorse version called the P32. Later, the W series chassis was designed strictly for motorhomes. Workhorse quickly garnered market share in the industry be committing itself to improving the Breed. What began as a W20 chassis has now grown into W22, W24, and W25.5 variations. The powerful 8.1 Liter GM Vortec V8 is coupled to Allison transmissions and is now found in every Workhorse gas chassis. In addition to the popular W series gas powered chassis a rear diesel R series chassis and rear engined UFO series chassis were introduced (although not as popular as the W series) and Workhorse has now entered the class C market with a Chevy based chassis as well.
So anything 1999 and up with a 7.4 L motor