I carry dirt bikes in the van all the time. In fact, dirt bikes and mountain bikes are the only reason for my van. I also sleep inside the van with the bikes. If the bikes are fuel injected, gas fumes are not an issues.It’s 100% going inside haha. But I do understand your concerns! Maybe a safety strap to the rear would be a good idea just in case.
I use a full length cross bar to chock my bikes. I also run a heavy duty strap from the rear d-ring to the foot peg. I am off all summer so I travel very long distances with bikes in the back. Keeping them from coming forward in a collision is a real concern.