The 15 ft sewer hose will likely do the job about 90% of the time in established campgrounds. For the other 10%, you'll be really glad you bought the extension.
For drinking water, the same holds true for a single 25' hose, so buy two of them. I would also suggest having a suitable 25' power cord extension on hand, along with a 30/50 or 50/30 adapter, depending on which supply your new TT uses.
A surge protector is an excellent safety device to use, as is a water pressure regulator. Water filter use will depend somewhat on where you plan to camp, as well as whether you simply want to remove the chlorine taste, no matter where you are. Most campground water supplies are safe enough, so generally it's just a matter of taste and sediment control.