There are no downloadable manuals that I am aware of. You can start with R1200GS How To Projects
for some general maintenance functions. There are also do-it-yourself sites that have pictures of many common maintenance items. Example: JVB Productions
You can also find tutorials for some functions by searching advrider.com. In your search bar enter "site:advrider.com" in addition to you search terms to limit the search to that site.