MAX485 controlling output voltage
There is no need to raise Vcc above 5V for your application. You must be looking at the "no load voltage".

This chip drives the differential outputs with an output resistance of 41 Ohms (typ)
 41 Ohms =  Voc/Isc = 3.7 V / 90 mA. With this, it can guarantee the min. standard 1.5V differential into a bus termination of 54 Ohm with up to 128 Rx ports on the bus.

This IC has common mode Tx voltage is 3V with no load and 1.5V loaded with +/- 0.75V minimum guaranteed.  (anything more is "gravy" , too much, >12V
 will damage the IC.)


Again, nothing you have said indicates you need Vcc=12V. ("Absolute Max" Do not exceed)  

That is for cases with long (up to 2km) cable loss and possibly some active terminator voltage.


  If you want to improve signal integrity, use the correct cable and impedance.


You do not not need to have more 0 to 3Vdc on the bus.  

The threshold for data 0,1 is 0V differential. This balanced approach with termination R gives a high Signal to Noise ratio for good signal integrity.