Good proven system high amperage solenoid and heavy cable.
one switch to bypass and ignition on to activate.
Basically run the solenoid between the two positive terminals.
To activate the solenoid use ignition on wich will join the two batteries together when the ignition is on leaving them seperated when it's off.
And have a switch in cab going to earth this will be the bypass or disconnect the two even when the ignition is on.
Simplest design and it works like you would expect.