Problem solved. Got the drawer out. That was the hardest part. There is a white plastic tab on the right side of the drawer. Needed to press the side inward with a small pry bar to release, while pulling drawer out.
Took off cap. There is a spring and thin circular diaphragm. Seems to be stuck open as nothing moves up or down. Not getting a new one. Taped the two holes very well, applied pipe dope to threads and it is back in. Odor seems to be gone. Will know better tomorrow.
Pipe cap made by J+B Products in S. Bend, Ind. PRO-VENT PV 140. ABS-1. It is 1-1/2 inch, for anyone interested.
2003 Dolphin LX 6355 w/ W22 chassis; 8.1L gas & Allison 1000