After you've created an Email Template that includes the one key merge field {!Campaign.EmailBodyHTML}, any content you placed in the WYSIWYG editor on the Edit HTML Body page will be merged into your Email Template.
To see the content that would be merged into your Email Template for a Campaign, go to your Campaign record and click the "Edit HTML Body" button. The subsequent page will display all the content that will appear in your Email Template.
Happy emailing!