Both are your model info and both can be used. The WPGR38 indicates that is a Workhorse chassis and not a Ford chassis. The 38R is your floor plan designation.
The 38R only came with the Workhorse chassis but it will always help to mention that info to be sure you get Chevy, Alison and Workhorse chassis parts not Ford which was standard on other Adventurer models that year.
I’ve been told that Workhorse changed the chassis etc in 2005. So your year is vital when ordering parts. There is a Workhorse chassis forum on
www.irv2.com. You might spend some time there looking at info.
Here’s a link to your brochure, the specs are on the last page.
https://www.winnebago.com/Files/Imag...nturer-bro.pdf