The Big Boy Battery isolator **IS** the Solenoid.
It should have two large (like quarter inch) and two small (like 1/8 ) posts
The two big ones are house and chassis battery
The two small ones control
To test your control switch. Hook a TEST LIGHT across the two small ones. (NOT AN LED but a regular old test light.. Don't have one.. A dash indicator lamp (Wedge base) and a pair of jumper leads make sure you don't short the leads at the lamp) If it is 3 position spring return to center Use/Store or however they label it Lamp should light either way you push the switch .
If it is an on/off toggle. Lamp should like most likely in ON position.