There is no such thing as a Discovery "repair manual" - Fleetwood never produced one or anything like it. There should, however, be a Freightliner chassis manual for a 1998 XC chassis.
There is a set of chassis owner help manuals on the FCCC website. Scroll down this page and find DOWNLOADS, CHASSIS MAINTENANCE, and the Know Your CHassis download button.
https://www.fcccrv.com/owners/#know-your-chassis