Okay, so I made this post over a year ago, and I have to say, the more I think about it, the less RTTE and the whole Dreamworks Dragons TV series seems canon. Not only are their a lot of inconsistencies that prove the TV Series is its own separate thing, but the characters seem different as well.
For example, Hiccup in RTTE was a tactical genius and possibly the smartest person in the entire archipelago. Think back to Defenders of the Wing, Part 1, where Hiccup cleverly determines that Viggo didn't actually want a truce and was trying to hide something on the island closest to the line on the map. Meanwhile, in THW, Hiccup was stupid enough to abandon Berk without even making sure if The Hidden World was a real place to begin with (remember, The Hidden World was nothing more than a "Mariner's Myth" before Hiccup and Astrid found it).
Another example, Astrid seems different as well. In the RTTE episode titled "Snuffnut", Astrid seems to look forward to the idea of marrying Hiccup, even going as far as to wonder why Tuffnut wouldn't look forward to it as well. She says, "Don't you ever want to meet a beautiful Viking girl and have a family of your own?" while holding onto Hiccup. In THW, she seems to be afraid of the idea of marriage, even leaving the Great Hall when Gobber brings it up. This one could be attributed to her just having cold feet, but it was still strange to see.
With Snotlout, we see his character develop across the series, from someone who antagonizes Hiccup and often undermines his authority, to someone who trusts and respects Hiccup as their leader. Then in THW, instead of getting the developed Snotlout we saw in the show, we basically get old Snotlout who thinks he'd be a better chief than Hiccup, even hoping for Hiccup to fail as Chief, so that he could take over.
There's a lot more, but this is long enough as it is. I genuinely believe that the TV Series and all the shorts are a separate canon to the Movie Sequels (HTTYD 2 and HTTYD:THW). I like to think of RTTE as being similar to Star Wars Legends, while the movies are more like the Star Wars Canon. The first one isn't the main canon, but is the superior franchise.