I have a lot of experience with this. I built the T-Rex range extending trailer as a dual use device. Its a standalone unit that I can use to power my car, power my house or take it to a cabin for the weekend. Lots of uses.
I just finished building T-Rex V2 that is a camper on top of a battery. That version includes a 3000 watt dc to ac adapter. Perfect for running power tools in the forest with no power around for miles. As long as I leave myself enough range to get home, I could go into a forest, cut it down and leave a month later. haha
You would need an inverter that can handle HV in. There arent many. I found one locally from a wind installation company.
Yes your car would need to be on in ready mode if you planned on tapping the battery in the car. Its the easiest and best way. When it is on the car will maintain the temp of the battery. If you were CAN savvy you could possibly figure out what the car needs to see in order to close the battery contactors but I think that is a long hard battle that you would lose.
The best place to tap into the battery is at the junction box at the battery itself. However if you have Jdemo, you wont have any room because thats where QCP taps into it. They then bring it to their junction box under the hood.
I could help you if you need anything.