Triac(s) as SPDT switch in a DC environment
Once you turn on a triac you cannot turn it off without removal of power. Triacs work on AC quite happily because they are "commutated" by the cyclical nature of the AC waveform. Hence you can activate a triac at some phase angle and it will turn off at 180 degrees or 360 degrees hence, you get a basic dimmer switch.

With a triac on DC there is no means (that I'm aware of) of turning it off.

That will end up with melt down in your circuit because all the contacts will be on and you have a big short circuit.

You say you don't want power loss but the series volt drop of a triac will be naturally much bigger than a decent sized MOSFET. MOSFETs, I believe, are the likely way to go. Look at the data sheet for the parameter V\$_{TM}\$ for your triac - it is quoted at 1.55 volts at a conduction current of 60 amps: -



That's equivalent to an on-resistance of 26 milli ohms. Many MOSFETs are available that are sub 5 milli ohm. 

If you are running a load that is 5 kW, that's a current of about 400 amps (at 12 volts) and a phenomenal power dissipation for the triac. Just look at the graph.