F. W. Danker (BDAG 3rdEd.) under ἀληθῶς cites Pl.Phd. 129e (sic) actually 109e-110a in support of reading GspJn 1:47 ἀληθῶς Ἰσραηλίτης as an attributive use of the adverb ἀληθῶς.
Plat. Phaedo 109e-110a
γνῶναι ἂν ὅτι ἐκεῖνός ἐστιν ὁ ἀληθῶς οὐρανὸς καὶ τὸ ἀληθινὸν φῶς καὶ ἡ ὡς ἀληθῶς γῆ.
… recognize that is the real heaven and the real light and the real earth.
GspJn 1:47 εἶδεν ὁ Ἰησοῦς τὸν Ναθαναὴλ ἐρχόμενον πρὸς αὐτὸν καὶ λέγει περὶ αὐτοῦ· ἴδε ἀληθῶς Ἰσραηλίτης ἐν ᾧ δόλος οὐκ ἔστιν.
When Jesus saw Nathanael coming toward him, he said of him, “Here is truly an Israelite in whom there is no deceit!” NRSV
The attributive (adjectival) use of the adverb ἀληθῶς in Plat. Phaedo 109e-110a is unambiguous since ὁ ἀληθῶς οὐρανὸς is parallel with τὸ ἀληθινὸν φῶς. Guy Cooper 1:50:8:4 calls this “virtually adjectival” but without really losing the adverbial force(??).
In GspJn 1:47, however, there is no article with Ἰσραηλίτης, furthermore ἀληθῶς is used adverbially several times with ἐστιν in GspJn and we never see ἀληθῶς between the article and the substantive in the NT.
John 4:42 τῇ τε γυναικὶ ἔλεγον ὅτι οὐκέτι διὰ τὴν σὴν λαλιὰν πιστεύομεν, αὐτοὶ γὰρ ἀκηκόαμεν καὶ οἴδαμεν ὅτι οὗτός ἐστιν ἀληθῶς ὁ σωτὴρ τοῦ κόσμου.
John 6:14 Οἱ οὖν ἄνθρωποι ἰδόντες ὃ ἐποίησεν σημεῖον ἔλεγον ὅτι οὗτός ἐστιν ἀληθῶς ὁ προφήτης ὁ ἐρχόμενος εἰς τὸν κόσμον.
John 7:26 καὶ ἴδε παρρησίᾳ λαλεῖ καὶ οὐδὲν αὐτῷ λέγουσιν. μήποτε ἀληθῶς ἔγνωσαν οἱ ἄρχοντες ὅτι οὗτός ἐστιν ὁ χριστός;
John 7:40 Ἐκ τοῦ ὄχλου οὖν ἀκούσαντες τῶν λόγων τούτων ἔλεγον· οὗτός ἐστιν ἀληθῶς ὁ προφήτης·
John 17:8 ὅτι τὰ ῥήματα ἃ ἔδωκάς μοι δέδωκα αὐτοῖς, καὶ αὐτοὶ ἔλαβον καὶ ἔγνωσαν ἀληθῶς ὅτι παρὰ σοῦ ἐξῆλθον, καὶ ἐπίστευσαν ὅτι σύ με ἀπέστειλας.
Danker cites John 8:31 as another example but it suffers the same ambiguities as GspJn 1:47. I don’t seen any compelling reason to read it as anything other than an adverb with ἐστε (c.f. D.A. Cason Gospel John Pillar PNTC), but I am open to suggestions.
John 8:31 ἔλεγεν οὖν ὁ Ἰησοῦς πρὸς τοὺς πεπιστευκότας αὐτῷ Ἰουδαίους· ἐὰν ὑμεῖς μείνητε ἐν τῷ λόγῳ τῷ ἐμῷ, ἀληθῶς μαθηταί μού ἐστε
C. Stirling Bartholomew