[AUCTeX] (La)TeX server fill

[Rodolfo Medina]

Hi.

I'm a plain TeX user, although in some documents I use commands like
\subchapter and so on. Then AUCTeX believes I'm using LaTeX and so, right down
the Emacs buffer I can read `LaTeX server fill'.  In other documents, without
those commands, there is instead `TeX server fill'.

How can I force AUCTeX to `LaTeX server fill' at my pleasure?  I need some
features that come with the `LaTeX server fill' mode.

Thanks for any help
Rodolfo