Vogler

In two examples of his Gründe der Kuhrpfälzischen Tonschule in Beyspielen (c 1778), Georg Joseph Vogler makes use of Roman numerals to show the fundamental basses of cadences. The first is in Tab. XXI, illustrating ten different cadences:

and the second in Tab. XXVII, illustrating the 7th chords utilized in cadences:

In his Handbuch zur Harmonielehre (1802), he mentions (p. 4) that he writes the fundamentals (Grundtöne) in Roman numerals; but in the book he makes only a limited use of them.
He uses them more regularly in Zwei und dreisig Präludien für die Orgel und für das Fortepiano (1806), where at times he compares the real bass with the fundamental bass, for instance (p. 30):

N. Meeùs