Connect 12v DC sensor to Arduino UNO using TTL RS485
You should be fine.
RS-485 does specify signal levels of ±1.5V to ±5V, but even so, and even when the sensor would misbehave or when the common mode voltage is low or high, there is still the TTL-to-RS485 converter in the middle that has 5V TTL logic on the Arduino side.
As long as the TTL-to-RS485 converter is properly connected to the Arduino and working as it should, the Arduino is safe.