I played around and you seem to able to strip out the tags from the html string the richtextbox exposes with the following:
Concat(AddColumns(Split(RichTextEditor1.HtmlText,"<"),"NewResult",Mid(Result,Coalesce(Find(">",Result),0)+1)),NewResult)
If you wanted to then prevent submissions from going over 375 characters, you could set your DisplayMode on your submit button to disabled if the length of the above formula goes over that limit.
If(
Len(Concat(AddColumns(Split(RichTextEditor1.HtmlText,"<"),"NewResult",Mid(Result,Coalesce(Find(">",Result),0)+1)),NewResult))>375,
Disabled,
Edit
)