A few questions:
When you say template, do you mean workbook, Sheet, Dashboard?
Are you trying to add the visualizations for campaign 2 to the same workbook as campaign 1?
Are all of the fields being used the same in the campaign 1 and 2 data sources the same?
if the above answers are Dashboard, Yes, Yes then the following workaround should be much quicker
Duplicate the campaign 1 workbook
Add the data source for campaign 2
Replace Campaign 1 data source with Campaign 2 data source
save this workbook as campaign 2
Open the Campaign 1 workbook and select File>import workbook
import your campaign 2 workbook
this will bring in all of the dashboards, sheets, stories and Data sources.
hope this helps, if not please clarify where I have misunderstood
Tableau does not like to change the connection itself typically.
I recommend that you prepare other database to store the historical campaign data as "Master Campaign data" or something.
Tableau always to connect to this data source and you can add new campaign's data to this data source.
You can use SQL server or Alteryx or something to manage database.
I don't recommend to use "Append" because it easily lose the version tracking.
thank you very much! The "Replace Data Source"-feature was exactly the one I searched for!
To answer your question: I wanted to create a template-file, consisting of a Story, many Dashboards and even more Sheets. I wanted to use exactly the same template for every new campaign, but it was very annoying to change every data source of every Viz on every sheet, so the "Replace Data Source"-feature is exactly what I needed.
Just one note.
Replacing Data source itself is very tricky functions.
Refer attached link to understand potential trouble.
thank you for your answer! I appreciate your concerns and will keep them in mind if there are any problems with this solution in the future.