Looking for an obscure book about time travel.

I remember reading this book quite a few years ago. I was in Junior high when I read it. I don't remember the title or author but I got a basic idea of the plot and I really hope someone can help me find this book.

Some kid finds or is given a device that can allow him to travel back and forth in time. I remember there is also a girl but I cannot remember either of their names. The girl's father wants the device or has one of his own, I cant remember.

I'm sure at one point the kid travels to the future and gets a rocket pack or something of another and it describes the future city in great detail. The girls father or someone tells them that every time they travel through time they create another universe or timeline. For some reason it is dangerous. He kidnaps the guy and the girl and brings them to 40,000 BC, I think. Somewhere around there anyways. He had a camper trailer and an SUV or some sort of vehicle.

I think the guy and the girl kill him in the end, my memory on the plot is very hazy and if anyone can give me any info on the author or title I'll be forever greatful, this is driving me to insanity trying to track down the book.