Will RS-485 work over non-twisted pair?
Will it still be reliable if a section of the data line is not twisted
  pair?


It could be reliable or at least made reliable but remember that automotive and vehicle environments are pretty bad for both power supply cleanliness and interference.

If you only need slow data rates then it's quite feasible that if the first packet sent to a slave gets corrupted (or the return packet gets corrupted) this can be detected by incorporating checksums into the data stream (appended to the data payload) and a retransmission performed.

I would also consider it sensible to use RS485 interfaces that are galvanically isolated such as this one: -



LTM2881 in more detail: -



See the galvanic isolation barrier in grey. The barrier allows much higher common mode interference levels on the RS485 cable i.e. it is a more robust solution but does come with a price hike.

There are also offerings of a similar nature from TI, Analog Devices and possible Maxim.