Data about anime adapted from video games does not always separate out genres as you would like (you seem to be primarily interested in RPGs, but data about anime adaptions may generally group RPGs along with other games such as children's games, dating sims, visual novels, etc.), but according to page 8 of Anime and the Visual Novel: Narrative Structure, Design and Play at the Crossroads of Animation and Computer Games by Dani Cavallaro (Jefferson, NC: McFarland, 2010) and Manga, Anime and Video Games: Between Adaptation, Transmedia Extension and Reverse Remediation by Stefan Werning,
manga and anime based on digital games appeared much later [than video games]; for example, series like Fate Stay/Night (2006) have been developed as adaptations of existing visual novels. . . . Sword Art Online (2009-2014), Accel World (2012) and Btooom! (2012) is constructed around a fictional video game. This phenomenon can be partially explained in the context of films referencing (digital) games as plot elements or even as narrative structures. . . . adaptations of popular game franchises such as Super Mario Bros. (Rocky Morton & Annabel Jankel, 1993) or Street Fighter (Steven E. de Souza, 1994, Figure 2) were first produced for an international audience [in other words, not produced for Japanese demographics].
And according to Mélanie Bourdaa's "‘Following the Pattern’: The Creation of an Encyclopedic Universe with Transmedia Storytelling," (Adaptation 6, no. 2, 2013: pages 206 and 211) and Werning,
Even though many of the anime series represent adaptations of light novels, the games usually do not cover the “canonic” stories from the novels but rather pick up loose ends and elaborate on the characters, often in unusual ways, thereby adding to the “multiplicity” of the franchise.
Wikipedia's List of anime based on video games evidences that the number of anime adapted from video games has been dramatically increasing within the past decade. By "full-length" anime adaptations, if you mean a TV anime of any length, including the current trend of series that are 10 to 12 episodes in total, the number of these has been greatly increasing as compared to the situation in the 90s or the 80s. It is not an overstatement to predict that the percentage of all anime produced which are video game adaptions is likely to rise in the next decade.
See senshin's answer to Are anime that are not based on manga common?: during the 2000s, anime adapted from video games have maintained a sizable chunk of all TV anime produced and has increased in percentage since 2006. You can see in the graph that video games have gotten adapted into anime more often than visual novels have. See also Why does anime air in 4 seasons per year of ~13 episodes? and What are the reasons for producing short-form anime?