Tl;dr: he's probably real, but his exile may have nothing to do with guiding the Persians.

First, many Greeks fought for Xerxes. There were many more Greeks in the invasion force than there were in the defence force. Many locales "medised" and contributed strength to the invasion, and Trachis may have been one of them.

As to Ephialtes himself: he's named as Xerxes' guide in Herodotos 7.213; Pausanias 1.4.2; Strabo 1.1.17; and Polyainos 7.15. Diodoros 11.8.4-11.9.3 says that the guide was a Trachinian, but doesn't name him.

Of these, the two most detailed sources are Herodotos and Diodoros. Herodotos is also the only one that gives details of what happened to Ephialtes afterwards. Herodotos was writing about 60 years after the battle, all of the others centuries later; Diodoros' account is based on a lost history written a few decades after Herodotos. Basically, Herodotos is the only significant source.

One other source has a variation: Ktesias, reported in Photios Myriobiblon cod. 72, at 39.i.32-39, names the guides as Kalliades of Trachis and Tissaphernes of Trachis. He was writing around the same time as Herodotos, but is far more unreliable -- partly because the original text doesn't survive, partly because his narrative generally sounds much more made-up.

So, it's Herodotos that tells us the details about Ephialtes. He tells us he was Malian (the people of Trachis were part of the Malian ethnic group), and from Trachis. Afterwards, he fled to Thessaly out of fear of retribution from Sparta. The Amphiktyons (a union of local leaders in the region) put a price on his head, making Ephialtes an exile. At a later date, Epialtes came south and was killed by another Trachinian in Antikyra, in southern Boiotia.

Your suggestion of Spartan propaganda can't be ruled out: there certainly was a lot of Spartan propaganda about the battle. However, two points.

First: Herodotos (7.214) gives a variant, according to which the guides were Onetes of Karystos (in Euboia) and Korydallos of Antikyra (Boiotia). Herodotos doesn't believe this variant, he says, on the grounds that it was Ephialtes that had a price put on his head. The fact that he is willing to use this as a way of distinguishing between the two stories suggests that he's more sure of the testimony about Ephialtes' exile than he is about the testimony about Ephialtes guiding the Persians.

And second: a lot of cities in central Greece were conflicted over the invasion. Thebes, for example, had factions both for and against Xerxes, so it was only one Theban faction that assisted with the defence. Ephialtes' political obligations aren't at all clear.

Remember, at Thermopylai more Greeks were fighting for Xerxes than against him. Diodoros tells us that the Malians fought for the defenders, but "a Trachinian" told Xerxes about the alternate path. In Ktesias, all the Trachinians fought for Xerxes. Herodotos doesn't mention the loyalties of Trachis or the Malians generally.

(Note on the numbers: according to Herodotos, the defending force during the battle consisted of 700 Thespiaians, 400 Thebans, 300 Spartiates, and an unspecified number of Spartan non-citizens and slaves: so, maybe 1500-2000 in all. In the invasion force, he tells us, there were 300,000 Greeks -- Thessalians, people from Asia Minor, and perhaps others. Now, that number is totally implausible. He gives a figure of 2 million infantry for Xerxes' army, and that's a ridiculous number. Even if we take it that the numbers are exaggerated by a factor of 10, though, the Greeks in Xerxes' army outnumbered the defenders by more than an order of magnitude. To have more Greeks defending Thermopylai than attacking it, you'd have to assume an exaggeration by a factor of 200 -- but then the entire Persian army would only be barely bigger than the force initially stationed there, before Leonidas ordered a partial withdrawal.)

It looks to me most likely that Ephialtes was caught up in the factional politics in central Greece as to which side to take: he may have been one of the more prominent pro-"medisers". (Notice that even in the variant story that Herodotos rejects, it's two people of central Greece -- one Boiotian, one Euboian -- that guided the Persians.) And as Herodotos tells us, it was the Amphiktyons that ordered his death, not the Spartans.