Replaygain tag issue

Using Foobar 2000, I have scanned and tagged most of my music library (over 16,000 songs) using the "Scan as Single Album" feature. Ideally, I can then go into MP3Tag and convert the Replaygain information to SoundCheck for use in ITunes.

My problem is this: MP3Tag doesn't seem to see all of the Replaygain information that Foobar 2000 sees. In fact, MP3Tag only sees the Replaygain tag info for about a quarter of my library. Foobar 2000 sees the Replaygain tag info for all songs.
If Foobar 2000 can see the info, why can MP3Tag? Is there some different format that holds the Replaygain info that MP3Tag can't handle?

As it stands now, I will need to scan 12,000 songs for Replaygain information again so that MP3Tag can read those tags (even though Foobar 2000 sees them fine).
Any ideas what's going on?

Have you checked "View > Extended Tags" on these files? What's in the caption after "Tags - "?

It does not show any Replaygain tags in Extended Tags even though Foobar 2000 thinks they're there.

In the caption after Tags, the majority of them say ID3v2.3 (ID3v1 ID3v2.3 APEv2). I generally make sure all my songs are tagged with ID3v2.3. I'm not sure where I pick up the ID3v1 or APEv2 tags.

So the RG info is in the APEv2 tag which Mp3tag by default does not show.
Usually APEv2 comes from the mp3gain tool.
I think it's best to get rid of the APEv2 tag and merge it with the ID3v2 tag:
In foobar2000, right click on a file > Tagging > Mp3 Tag Types and deselect APEv2 (and ID3v1 if you want).

Just a heads up in case you didn't know unless you go into the options in Foobar and turn on Compatibility Mode Foobar will save MP3 tags as ID3 v2.4.

Thanks for your help. I deselected APEv2 in Foobar, but that appears to just delete the APE tag. Is there a way to merge the RP info in the APE tag to the ID3v2 tag?

Well, I couldn't figure out how to do this with Foobar. So I tried using MP3tag to move the RP info in the APE tag to the ID3v2 tag. The only way to do this is to copy all the APE tag info to the ID3v2 tag, which screws up much of the tag info (such as album art). It would be nice if MP3tag allowed individual tag fields to be copied from one type of tag to another.

In any event, I just deleted all the APE tags, and now I'm rescanning all 12,000 songs in Foobar (this will take 17 hours). Once that's done, I can create the iTunes tag, and reimport the songs into iTunes.

You were right, foobar2000 can't copy the APEv2 tag over this way, my test file also had ID3v2 RG tags so I thought it worked.

There is a method that seems to work:
In the Preferences, go to "Advanced > Tagging > MP3" and select
[x] Force preferred tag writing scheme on all files regardless of existing tags
Then you need to select your files, right click, Properties, add some dummy tag field with a value and hit Apply.