My 2015 XLR has that extra button on the panel, too, but I don't have a second gray tank. I'm guessing FR just bought a boat load of the same panels and used them in all the XLRs, even if that 5th button wasn't needed. So don't assume you have a second gray tank just because you have an extra gray tank indicator button...
2015 XLR Hyperlite 30HFS5 (mods being performed regularly)
2009 Salem LA 292fkds (gone)
Nights- ('12)23 ('13)23 ('14)15 ('15)31 ('16)25
2014 Harley Davidson FLHX (XLR cargo)
2014 VW Jetta TDI (diesel sipper, camping support vehicle)
2011 Ram 2500 CC 4X4 CTD, B&W Companion (toy hauler hauler)