Lews 2018

Good Day Good People I have been following various forums looking for the right dual battery solution. Since I didnt want to trust an aftermarket solution, I went to the dealer and had them retro fit the complete wiring for the 2500 series silverado. Everything went in and appears as a stock solution but there is a flaw. There is no way to send the signal to open/close the relay to charge the second battery. The mechanic gave me 2 options; connect the power cables bypassing the relay (one power cable goes to the alternator the other goes to the second battery) put a on/off switch inline with the harness (see attached image) along with an amp gauge to monitor usage. Any other options? Thanks for the assist..