For some time now, the s-apostrophe-s as in "Truss's" has been the
"prefered" form, both in writing and in pronunciation. I can't quote you chapter and verse off the top of my head, but I think you'll find this in any reputable book on style from the last thirty years or so, at least. I have a feeling that even Fowler, back in the '20's (prefered modern style: "20s"), recommended this usage.
Just checked my 1999 so-called "Fowler's", and there is a little more ambiguity than I have indicated above; to wit: "Add 's to names that end in s when you would pronounce them with an extra s in speech (e.g. Charles's, Dickens's ... ); but omit 's when the word is normally pronounced without the extra s (e.g. Bridges', Connors', Mars' ... )." Not terribly helpful, is it?