Thanks to "F6C", I have discovered the Triumph parts instructions site 
You can find there also how to install the "front mounting harness". I understand now why the Triumph main site reports 18 minutes needed to install the tank bag.
Get the part number from the main site e.g. for the bag : part nbr A9518171 (you find it there : Accessorise your ride)
Then go to the Triumph parts instructions site : Triumph Instructions.com
then type in the part number (top left of the screen "Part No."), click "Search" and select your language and it'll result in the PDF (6 pages) e.g. in English : https://www.triumphinstructions.com/ProdDocs/A9518171-US.pdf
When I look at the PDF, it looks like that you don't have to remove the bag to put gas 
