I know you posted last year but I found your post because I had the same issue- RGB and still no HDR toning.
Just a bit of an oversight in Photoshop- HDR Toning will be greyed out if you have the wrong tool selected!
I switched to the Move Tool and the HDR came back!
NOTE: Photoshop will not tell you that you need to do this, it's kind of a secret
Hope the thumbnail worked out OK!