I am going to attempt to extend the run time om my RX450H accessory mode.
I have the RES option. We can watch a movie while the Wife shops.
The run time on the car is like 20-30 minutes and the car displays a message it's going to shut down to conserve power
I don't know if this timed or based on the 12v battery voltage.
Tomorrow I am going to back feed through the cigarette lighter a 18ah battery. It's a Lithium Iron.I will monitor the battery voltage and see if this additional battery shares the load.

Let you guys know what happens.