On my '17 Triton there was a button on the gas line that you had to press after changing the cylinder or turning on the gas.
My 04 Troll doesn't seem to have the same feature but the manuals are completely silent about this process.
So, as far as I can tell as soon as you open the valve on the tank gas will flow straight to the appliances.

Is that correct?

Also, does anyone know what the little valve thingy under the plastic screw on cap left hand side is for?

Thank you.

Looks like a straightforward gas regulator, connect and go.

The little valve could be a test point?