2010 1200 Sportster, although 100+ miles fuel

Ride the bike until it starts "holding back" or whatever it does. If you think it's fuel level related. Disconnect the fuel line at either the carb or the tank. Turn the fuel on, or if you have one of those vacuum operated petcocks, apply vacuum with a hand operated vacuum pump to open the petcock and test the fuel flow. See if you have a full stream of fuel coming out of the hose. Then fill the tank, and retest. Notice the difference in the flow.

Now, if there is a noticable difference, what is causing it. It could be the petcock screen. I've seen the screens have a "too high" standing pipe for the main fuel supply. The screen has a "pipe" made into it that is usually about an inch above the petcock. Fuel flows down this pipe when the petcock in the "ON" position. When the fuel level in the tank falls below the height of this pipe, the engine sputters and sometimes stalls indicating that you need fuel. But, when you switch the petcock to "RES", another hole in the petcock, which does not have the standing pipe, will open up and allow you to use the remaining fuel in the tank. On a Sporty, this is usually somewhere around a gallon of fuel or about 40 miles or so.

One other thing that could be effecting this the level of the bike. If they has been lowered this could have altered the "level" of the bike. On a Sporty with it's small capacity tank and the shape of the tank, if the has been lower by as little as 1" in the rear, it will cause a problem such as this.

I notice that the bike is a 2010 model. If it's still under warranty, take it back to the dealer and let them figure it out. That's what a warranty is for. But, just in case, keep this things I've mentioned in mind.

Good Luck
Steve