RC Lawnmower: Joystick emulation: RC PWM to _custom_ DC conversion
The "path of least resistance" using "something off the shelf" would be to couple a standard R/C servo to a potentiometer (like was done in early R/C speed controllers). Center position and end-points can be adjusted mechanically, or via the R/C transmitter. 

Most pots have 270º rotation, while most servos do about 90º. If the pot is powered from 12V then to get 4.6~7V You need about 20% of the pot's range, which is 54º. To get this reduced range you just have to dial the TX end-points down to ~60%. If your TX doesn't have end-point adjustment then you could wire padding resistors in series with the end terminals of the pot, eg. a 5k pot would need about 1.7k in each leg (use 2k trim pots for fine tuning).







simulate this circuit &ndash; Schematic created using CircuitLab