You're probably expecting too much for a superzoom lens. They tend to exhibit more autofocusing inaccuracy at the long end of the zoom range. Additionally, because of some of the design tradeoffs required to achieve such a large focal length range, the optical performance tends to suffer at one end or the other (usually at the long end).

Regarding autofocus accuracy, quoting from Nasim Mansurov's review of the 18-300mm at Photographylife.com,

As you zoom in, however, autofocus accuracy is inconsistent and can be all over the place – with plenty of hits and misses. Anything above 105mm tends to miss focus and it gets worse at 200mm and 300mm. With such a complex lens design, I can see why it is so weak on the telephoto side. [...] Well, that’s what you get with a superzoom. Focus tracking in OK in continuous mode, again only at short focal lengths. When the lens cannot autofocus and starts to hunt, the autofocus performance gets to a crawling speed. I was able to get a couple of sharp shots at 300mm, but it was not easy. I had to constantly refocus and take pictures and eventually got a couple of keepers.

Later in the review, commenting on the Imatest sharpness scores (MTF performance) at 300mm,

At 300mm we get the worst sharpness – image quality suffers pretty badly and the resolving power of the lens is greatly diminished. These results are pretty typical for a superzoom though.

DPReview.com reached similar conclusions:

Our biggest concern about the 18-300mm, though, is its performance at the telephoto end. In practical use, chances are you'll be shooting at maximum aperture much of the time, and relying on the lens's VR system to keep things steady. But image quality at telephoto isn't great especially wide open, and this is compounded by a VR system that we've found to fail consistently across a specific shutter speed range (approx 1/125 - 1/40 sec). The D3200 we used for testing also frequently misfocused slightly when shooting at telephoto. This means that we often found real-world results at to be disappointing in the telephoto range - and especially at 300mm.