Fossil groups are galaxy structures that probably underwent a nearly complete fusion of all intermediate magnitude galaxies into a single large central dominant galaxy. However, the formation and evolution processes of these structures are still not well understood. In order to test this scenario and its implications we studied the fossil group RX J1119.7+2126, based on available spectroscopy of the galaxies in the low-density, large-scale region around the fossil group and deep B and R band imaging of its close vicinity and three comparison fields. We used spectroscopic data to investigate the degree of isolation of RX J1119.7+2126 in terms of bright neighbor galaxies. The imaging data were used to derive the color-magnitude relation and select faint galaxies statistically belonging to this structure. The structure appears as a very isolated group exhibiting a red sequence in the color magnitude relation with characteristics close to the red sequences already observed for other fossil groups. All these results can be interpreted consistently in the framework of the building-up process generally proposed for fossil groups.