Just a heads up, if you're not already doing so or not aware, you can semi-'test' your json coding thru jsonlint.com, and it will let you know if your coding is proper. Worth doing after each show/movie/item.
Granted, it won't let you know if it matches Roku's schema per se, or if its the right video/thumbnail etc, but at least tests for things like missing comma's & brackets etc, although you're editor likely also notifies you of things like that. Just thought I'd mention it.
Let me add to what @37mediagroup posted. The software I wrote to manage feed files contains a function to "validate" your feed file for not only syntactical correctness, but, for logic errors that a linter will not catch. And, it's free.
FREE Windows desktop software for converting Direct Publisher channels to SceneGraph (SDK), for creating BIF (Trick Play) files, Roku (MRSS, JSON) feed files, and FireTV feed files @ GitHub/rrirower.