You should have 11x17 schematics in the back of your Owner's Manual with the waste plumbing. I just looked up the 2006 m.y. schematic for a 34' rig, and it shows "Adapter, 1-1/2" Trap Plug" for the shower drain adapter. the drawing looks like its a threaded fitting.
There should also be an 1-1/2" threaded trap adapter below that where the vertical section of shower drain pipe enters the top of the P-trap.
The 2005 Owner's Manual from the Tech Library
shows the same 15331 part# for shower drain adapter, and 14321 part# for threaded trap adapter, so yours should be the same.