First I am not sure why you have two house battery banks with a selector switch. But anyway:
assuming that the round trip (pos and neg) is 2 x (17+7) - 48 feet and that the maximum continuous current is 373 amps as given in Magnum's specs, then you will need at least 3/0 to safely carry the current and 4/0 to keep the voltage drop below 1 volt.
So you do not necessarily have to change your routing, just upgrade your cable size. Genuindealz.com is a good place to buy boat cable. They can also crimp on connectors for your custom measurements.
But also consider the maximum AC load that you are going to pull from the inverter. If the maximum is only 1,500 watts then put a 300 amp fuse to protect the existing 2/0 cable and the DC required to pull 1,500 watts AC is only about 200 amps which will keep the voltage drop down to about 0.7. You can actually pull about 2,000 watts and keep the DC voltage drop to about 1 volt, which was probably the criteria for your old Freedom 20.
To summarize, the solutions are: different routing, bigger cable or only use 2,000 watts and use your existing cable.