Id3v1 supports up to 30 characters and ID3v2 is virtually unlimited.
The workflow you have used to remove ID3v1 tags should work.
In Mp3tag you should also have a tag column to see the tag types of a file or go to View > Extended tags.
I also think a car stereo is not a reliable tool to verify if a certain tag type exists or not. It may have bugs or limitations that we don't know.
One limitation might be incomplete ID3v2 support so try changing your ID3v2 settings like there:
/t/2785/1
If you don't trust Mp3tag that the ID3v1 tag was removed check what other tools on your pc show, i.e. try another tagging program or a player like Winamp.
Or the most reliable way is to use a HEX Editor and check the end of the file if there is readable text.