Dual battery/ignition question for a beginner

I had a dual battery setup recently installed. When I run a plug to my bed 120v outlet, and charge my phone / run a light strip on it...after about 20 minutes the car ignition shuts off. Then I have to go back in and press the start button twice to turn the ignition back on. I have a fuse box but nothing installed on it yet so maybe I’ll just run a cable to charge to that.
Why does the ignition shut off like that when using the bed outlet? It doesn’t do it when I run my other lights for hours at a time.

 

The ignition shuts off to protect the battery from discharge. If you want to be able to run the Inverter without having it shut off, you will need to rewire it to the auxiliary battery.