Is there an I²C device out there for reading a set of PWM inputs?
Thank you for linking to exactly what you are talking about -- it makes it much easier to get to the data sheets.
The question "How can I use more than one MaxSonar®-EZ1™ in the same system?" in the MaxBotix MaxSonar FAQ lists several ways of using a bunch of them at one time (and some pitfalls to avoid).
Some of them use 2 pins of the MCU -- the same number of pins as I2C, but a completely different protocol.

If I understand the data sheet correctly, one of those 2-pin methods is:


wire pin 1 low the MaxSonar to enable chaining
Wire an output pin from your MCU to the RX pin of the first MaxSonar in the chain.
Wire TX of each MaxSonar to RX of the next in the chain
Combine the signals from all the MaxSonar PW pins (AND? OR? NAND? diodes?).
Wire that combined signal to a timer input pin on the CPU.


Then software on the MCU


pulse the RX pin of the first MaxSonar
Time the first pulse width to give the distance the first MaxSonar sees
Time the second pulse width to give the distance the second MaxSonar sees
...
After getting the pulse from every sonar (or timing out at 49 ms per sonar), do something with the data.
repeat.


Perhaps one of the other methods listed in the FAQ would work just as well or better for you.