You're overlooking something. The Kindle Fire has only 8GB of storage! There isn't room for OS, apps, music, books, games, and your whole movie collection. There's barely room for ONE DVD at 4.3 GB if you're carrying much of anything else. Any movies would have to be really low Rez and highly compressed. . . Or streaming, so not much content on the road... Sorry.
DVDs are loaded with extras and use MPEG2 compression, which isn't as efficient as H.264 and other modern codecs, so the 4.3GB figure isn't terribly useful.
iTunes movies run about 900MB an hour. Assuming that Amazon's compression is similar, the Fire would hold 3-4 movies with a fair amount of room for music and books; if more highly-compressed versions are available, you could squeeze more in. If you're traveling with kids, that's about as long as you could reasonably expect to go without stopping anyway, and it wouldn't be too hard to find WiFi at any well-populated highway exit and reload. It's not seamless, but neither is carrying around a ton of DVDs.