Keith,
Congrats on being the first to start asking questions on the new Furuno USA Forum. If you PM your mailing address to me, I will shoot you a coffee mug for being the first to post.
With the FCV585, you can customize the ranges used under the MENU – SYSTEM – RANGE settings. By default, 1000ft is the max range, but that is easily changed. Just highlight one of the 8 customizable ranges (under Menu-System-Range) and hit the ENTER key to bring up the box to allow that range to be changed from 0 to 2500 ft. In your case you might want to use RANGE 8 for this ~2000ft depth you are looking for.
Now of course, if your range is 0 to 2000 feet it will be really hard to get a good close-up look at your targets. That is where some of the other features come in. You have many tools at your disposal but the two selections most useful for your case of getting into 300-2000ft range, would be SHIFTING, and Marker Zoom.
Shifting allows your viewing window to be shifted up and down to select starting depth. This is useful when you want to view a specific area say…for example 500 to 2000ft. The total range “window” you are asking to view at one time is 1500ft (2000 minus 500). So, you should change your overall range to 1500 ft and then SHIFT (MENU – SOUNDER – SHIFT) your window down 500 ft. This will give you a 1500 ft window covering from 500 to 2000 feet. So please note with your example of wanting a window from 300 to 2000ft isn’t possible using SHIFT because the increment range is linked to the depth range. The larger your range is set to, the larger the shift increments are. So at this depth level,the shift increments are every 500ft. So you can’t just shift 300 ft down. You would shift down 500 as the example above.
You still can accomplish your goal of seeing 300ft to 2000ft in a window via another method. That is by using the Marker Zoom feature. Please change your range to the setting of 2000 ft we previously setup at the beginning. Change your ZOOM MODE setting (MENU – SOUNDER –ZOOM MODE) to MARKER ZOOM. This will provide window next to your normal range that shows a “ZOOMed” in expansion of where your marker (cursor) is located. Since we need a very large area, you will have to change your zoom window range (opening) to the total range you are wishing to see expanded. In this case we need to see a 1800ft window (from 300ft to 2000ft). So I change my zoom range under MENU – SYSTEM – RANGE – ZOOM RANGE to 1800 ft. I then turn my MODE knob on the machine to “LF ZOOM” and move my marker (cursor) down 300 ft and the zoomed area at the left now shows 300-2000 feet (what you are wanting) and the right side shows the full range of 0-2000ft.
Please note: to do shifting you can not be in the AUTO RANGE mode. You must set your range manually.
Your question/issue with keeping bottom at 2000ft in auto needs some investigation. Are you running a 600W or 1000W transducer? Is the power setting (MENU – SOUNDER –TRANSDUCER) setup to match your transducer?
Maybe I will come up with something. Of course if I get bombarded with requests because of you, I will have nothing but dandruff to offer. 
