n. A state or condition characterized by solitude.
Van Vogt was the first Canadian sf writer of real importance, and it is arguable that a Canadian solitudinousness colours his work throughout.
—“van Vogt, A E,” The Encyclopedia of Science Fiction, October 14, 2014
Entering a relationship with a romantic “life partner,” commonly through marriage, is considered a significant social marker, denoting full adulthood and providing universally recognized social reasons, which do not meet with requests for further explanation. Some groups may accord this status only to marriage, others more broadly to cohabitants; but it is an evaluative social classification that friendships or, even more so, solitudinousness rarely breach.
—Elizabeth Brake, Minimizing Marriage, Oxford University Press, March 15, 2012
Just had some lunch. Just me and a book. Part of my new efforts at solitudinousness.
—Jon Becker, “Just had…,” Twitter, April 01, 2010
2007 (earliest)
The filmmakers ask how cinema and stars can help the Vietnamese people to win their independence. They must listen to these people and learn things. Their analysis of the photograph contrasts Fonda’s solitudinousness, her failure as actress to consider her militant activity, with the urgent reality of the Vietnamese male in the background.
—Dennis Grunes, “LETTER TO JANE (Jean-Luc Godard, Jean-Pierre Gorin, 1972),” Dennis Grunes, December 21, 2007
Isn't solitudinousness just the windbag's way of saying solitude? Not quite. That is, it's not referring to solitude itself, but to an inherent quality in some other thing that has solitude as its chief characteristic. Am I splitting hairs here? Very much so.

In case anyone asks you about the word solitudinous, "characterized by solitude," you can say that it is indeed very old, dating to at least John D. M'Kinnon's Descriptive Poems Containing Picturesque Views of the State of New York, published in 1802:

Diminutive beneath, the Hudson, deep
Coerc’d by rocks, and silent, penetrates
The solitudinous and woodland scene;