Using the 555 as V/F converter
An 8-pin micro with onboard A/D is a good solution. If there's already a micro (without A/D) in your system, it makes sense to at least use it for doing conversions (pulse-width to frequency, for ex) if it helps simplify the input circuitry.
For whatever reasons (learning curve, overhead), some people prefer not to use micros, and a simple analog/digital circuit can often perform the function well enough. The question you asked is a good one, if only for curiosity. The 555 is often used to perform the opposite function ( frequency-to-voltage (F/V) converter or tachometer) when connected in monostable mode.
Pin 5 (fm) connects to the upper node of the resistive divider in the 555, and allows you to vary the internal 1/3 and 2/3 VCC trip points a little. It's only meant for modulation over a small range, as you've found. Look at the description of pin 5
**broken link removed**. To vary the output frequency over a large range, I'd bypass this pin to ground and look for another way besides "pulling" the internal trip points.
**broken link removed** circuit comes close to what you want. It varies the output pulse-width based on input voltage. It's simple but has some problems, one being the 5V floor on input voltage.
Other ideas: replacing the upper resistor (R1) in the standard astable configuration with a simple V/I converter could work. FET's are often used as voltage-controlled resistors. They have some disadvantages, such as non-linearity, but may work for your application. Also, take a look at circuit #10 in the first url. It's a related idea, but just replaces the upper resistor with a one-transistor constant-current source (charges the capacitor linearly rather than exponentially).
Here are a few other V/F and VCO circuits (not using the 555)
**broken link removed**
https://www.electro-tech-online.com/custompdfs/2004/07/4584vco.pdf
(this one's a little cheezy, as it varies the supply voltage to the part to change oscillation frequency)
Finally, there are standard ICs that perform voltage to frequency conversion, like the LM331 and XR4151. Though most have been discontinued, you can still find them on the surplus market.