Often, various types of ciphers denote the real bass. They are then often combined with Arabic numerals indicating how the bass is to be figured. III6, for instance, denotes a chord of I (tonic) without indicating it as such. Early cipherings often are quite ambiguous in this respect.
One of the earliest examples of denoting the degree of the chords more or less independently of the particular key of the piece is in John Frederick Lampe‘s A Plain and Compendious Method of Teaching Thorough Bass, London, Wilcox, 1737. John Trydell, in Two Essays on the Theory and Practice of Music, Dublin, Boulter Grierson, 1766, utilizes at least once a similar system.
Johann Philipp Kirnberger, in the first volumes of his Die Kunst des reinen Satzes in der Musik, makes use of Roman numerals to denote either scale degrees or intervals. His usage is not clear, but may have suggested later ones, among others by Vogler.
Georg Joseph Vogler occasionally employed Roman numerals to denote fundamental basses in his Grunde der Kuhrpfälzischen Tonschule in 1778. He mentioned them also in his Handbuch zur Harmonielehre of 1802 and employed Roman numeral analysis in several publications from 1806 onwards.