If you run electric the ESC usually provides 6V. You need to get a BEC (like Castle BEC) and adjust the output voltage with that. In your case to 7.4V. The receiver will get 7.4V and so will the servo. You need to remove the red wire from the ESC's RX wires so you can "disable" its BEC. There are some pics on Castle's site.

On nitro you could get a LiFe receiver pack or a lipo one and again use the BEC to adjust output voltage.