My girlfriend should sleep in the car.
I don't understand. Why would you ask her to sleep in your car?
Because I want someone to sleep in the car with me and it has to be her.
While it is understandable that you want your girlfriend to share in an experience with you, making her sleep in a car will probably drive her away. You shouldn't force anyone to sleep in bad conditions unless they're willing to do it.
I love her and I want her by my side.
If you really love her, you would understand that it is not safe for her to sleep in the car. You should let her find a safer place to sleep until you can afford a place for you to sleep together.