This post is very old but I figured I'd answer since it took me a while to figure this out.
You can get the serviceable material in one of two ways:
Call function module IWOL_WV_READ_LIST and pass in the order number and item, and the serviceable material will be in the export structure T_RIWOL in field MATNR.
Or you first go to table SER02 where SDAUFNR = RMA order number and POSNR = RMA order item. Get the SER02-OBKNR field and use it to select a record from table OBJK where OBJK-OBKNR = SER02-OBKNR. The serviceable material will be in field OBJK-MATNR.