The battery boost solenoid on your '88 serves double duty. It is triggered by a wire that is hot when the engine is running, which engages the solenoid to allow the alternator to charge both banks. The momentary pushbutton does the same thing (likely through the same trigger wire on the solenoid); connects the two battery banks to serve as an emergency start if the chassis battery is dead/low. As "enblethen" says above, it needs to be a continuous duty solenoid.
Adding a switch in the line WILL eventually lead to both battery banks being drained and you being stranded when (not if) you forget to turn it off once you get to your campsite. It's an extremely simple and foolproof setup. Don't try to reinvent the wheel.
