For clues, see Daniel de Raulaudre's database at http://roglo.eu/roglo?lang=en;i=808261.
He lists as his source F-L. Jacquier, by which he means http://img.roglo.eu/~F-L.Jacquier/notices_externes/JACQUIER.pdf. No further information there, just an Ahnentafel.
Rauglaudre's supplemental sources are:
All of these agree that Pépin's wife is unknown.
The next step will be to check James Anderson's Royal Genealogies (1732):
This is on my list of things to do, but you might get to it sooner. Anderson is the ur-source for many of the royal genealogies we see in English. Finding a line in Anderson means that it was current in the 18th century. If a line is not in Anderson, it is perhaps a modern fake.
You can download the book as a .pdf free from Google Books, but you will find that it is pointless to use the search function. The download is variously available or not available, depending on the status of Google's licensing battles.
You can also check WorldCat to see if it is in a library near you: http://www.worldcat.org/search?qt=worldcat_org_all&q=james+ande...
Finally, you can also buy a copy. It is available in reprint (print on demand) for about $100, or in an original edition for between $662 and $1690 (currently). Probably worth buying an original edition if you're serious about medieval genealogy ;)
Are you saying Pépin isn't there either? If he's not, then we haven't learned anything. If he is there but not Rothaïde we need a harder look for just when she enters the world of European genealogy. We'll want to check the Marquis de Ruvigny, and others from the same period to see if they have her.
I'll bet you're right. She's not listed in Michel Call's Royal Ancestors of Some LDS Families, nor in Roderick Stuart's Royalty for Commoners -- two books that grabbed at everything they could find.
There was an LDS family who in the 1950s and later invented many royal lines. I can't think of their name right now, but their book was very popular for awhile in genealogy circles. She could be from that.
I have no objection to labeling her as fictional, leaving her marriage so we have a point of reference, but detaching her from any children so people don't repeat the mistake.
Your link doesn't get the full Turton, so I'll make a note to follow up and in Settipani's La préhistoire des Capétiens.
Shall we moved all the children to N.N. du Vexin?