In the year long trek we made to actually pull the trigger on our own Seneca, I ran into this problem a lot. The main answer was to take it to FL and make a warranty call. The answer is vague but has something to do with the fuel level sender in the tank. Most people indicate they just live with it and keep good logs on their fuel economy. Apparently, the RH tank holds all the fuel level sensors and are common parts to the M2 series of trucks and they do not calibrate the sensor to the tank it is married to all the time. Hope it helps.
2014 Seneca 37FS, 2012 Mazda2 on a Master Tow dolly