Hello Andrew,
Thank you for using our services.
Can you try to download the external templates for your version of Sitefinity. Then take the ~/Sitefinity/Admin/ControlTemplates/Generic_Content/GenericContentDesigner.ascx and turn off all content filters of the RadEditor:
Then map the external template of the Generic Content control with the following config line in the controls config file and restart the website:
More information on external templates can be found
here.
Regards,
Radoslav Georgiev
the Telerik team