Decoding a RS232 message in Arduino
I contacted MaxBotix with the problem and for my surprise their technical support is really great:


  The Arduino is set up to natively read TTL serial data.The MB7066 outputs its serial data in the RS232 format. Do not worry; this problem can easily be fixed. You can use an inverter such as the MAX232. You can also follow the following tutorial to make your own https://github.com/simonmonk/raspirobotboard/wiki/Rangefinder-Adapter.
  
  Another option would be to test using this code section SoftwareSerial
  sonarSerial(rxPin, txPin, true); //define serial port for recieving
  data, output from maxSonar is inverted requiring true to be set.