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 creating BIF (Trick Play), Direct Publisher (MRSS, JSON), and FireTV feeds @ GitHub/rrirower. No programming knowledge required.