As to the trees being unsharp:

It is very difficult to tell, but I think that the autofocus decided to get the house, not the trees in focus. It really is best to specify a certain AF point (p. 61 in the camera's manual). Or, if shooting from a tripod, use LiveView's freely movable contrast AF (p. 95, 102-106) - or focus manually (p. 98). My second guess, as nothing really seems that sharp, would be that it is due to lack of sharpness. Stop down your lens (start with f/8) and see if things get better.

In general, the 75-300 f/4-5.6 III USM is not considered to be very sharp:

If you care about great image quality and sharp photos, the Canon EF 75-300mm f/4-5.6 III USM Lens is not for you. And Photoshop cannot enhance details that are not there.

Source: the-digital-picture.com

The only positive feelings I have about [the 75-300 (Non-USM)] is that it covers the 75-300 mm range.

Source: chills42's answer on "How does the Canon EF 75-300mm USM III compare to the Canon EF 70-300mm USM IS?".

So if you have any other lens than that, it is perhaps best to switch to that as often as possible and/or to buy a decent lens (like - but not limited to - the Canon EF 70-300mm f/4-5.6 IS II USM).

Also, 1/30" could be a bit too long for trees - even very soft wind (soft enough to not be perceptible when wearing something long-sleeved) could move the leaves enough to become blurry because of motion blur.

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As to RAW vs JPEG: This has been covered at length here at photo.stackexchange. Take, for example, this answer on "Good examples of RAW's advantages over JPEG?" by @jrista:

The value of RAW is not really in the end result, although it is possible for the end result created with a RAW image to be better than that created with a JPEG. The reason for this has to do with the workflow between snapping a shot and saving or printing a final image. RAW gives you headroom that JPEG can't come close to offering. You have the ability to recover highlights and shadows, apply alternative tone curves, rework old RAW images with newer RAW processing algorithms to get better results, etc.

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As to metering:

p. 77 in the manual explains the differentiation between all the modes. As a rule of thumb, use evaluative metering as your default setting. It works well most of the time. I rarely use anything else, and if I do, I usually go for center-weighted average metering. In extreme rare cases, I used partial metering: That mainly helps when having extreme brightness differences between the background and your subject (e.g. black backdrop and flash-lit, white subject - or vice versa). I, for one, never seriously used spot metering.

As said, I usually (> 97% of the time) stay with evaluative metering. I usually either set the exposure myself (using M-mode) or let the camera decide on the ISO, and in the rare occasion that the metering is off, I simply dial in some compensation. With increasing routine, you might even learn to do that on-the-fly, e.g. when I see a dark background that takes most of the frame and a well-lit subject, I know that I need to set a negative exposure compensation. I could just as well use partial or spot metering, but then my subject would have to be in the center of the frame - and I would have to remember that only the center of the frame is evaluated (there is a reason why most cameras show an optional warning in the viewfinder when using spot metering). Your mileage on this may vary, but evaluative metering to me seems like a good default setting for anything, especially for non-studio shots.