[AUCTeX] how to configure the preamble used in preview

[Chengxi Yang]

hi,

The command "preview for environment" does not work well for my tex
file. I found this is because of the document class "thuthesis" I
used. When I use this document class, some errors appear, but
everything is fine when document class "article" is set in the main
tex file instead. I also found that the preview tex file
"_region_.tex" always use the same preamble with my main tex file.

So I'd like to know how to set a different document class for preview?
I found a little hint in the preview manual, such as using
prdefault.cfg, however, any more detailed instruction would be
appreciated.

Thanks in advance!

Chengxi Yang