As my colleagues already explained, RadEditor is an XHTML editor based on the rich-text editing engine of the browser.
This means that even when the RadEditor does not change the content (the ConvertToXhtml filter is disabled, as advised in the very first post from Rumen), the browser will attempt to fix the HTML content to make it valid.
Thus, it is the browser that changes the content and breaks your custom tags because it cannot parse them.
I am afraid that there is no workaround for this, apart from creating content that will pass XHTML validation and using that instead of the invalid content you have.
We cannot guarantee proper functionality with invalid content. Nevertheless, I tried your markup with disabled filters and the RadEditor behaves just like a simple editable div. I am attaching here a short video and a small sample that demonstrate this. If this does not suffice for your needs, I would advise that you move to XHTML-valid content.
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