Yes this is correct for what you are describing use a toggle switch, you can run a wire from the positive aux battery terminal (at the battery or at the solenoid) to the switch then from the switch back to the control terminal on the solenoid,
Most Motorhomes, come with an Aux/Emergency start button , which activates a momentary duty solenoid/relay , to connect the two battery systems for a built in jump start.
It Might be possible to replace that one with the new continues duty solenoid, then add the extra switch to turn it on , that would eliminate attaching more wires at the Diode isolator,
