You can use Calibre to convert to ePub, Mobi and other files. RTF is not a good format for ebooks. Different text editors show RTF files in a different way.

Calibre can be used to remove line breaks,... but it is not an ebook editor like Sigil.

If you can convert your ebook with Calbire, then you can be sure that the result ebook doesn't have any DRM protection.

If you have problems with Calibre, you need to post the exact error that you get.