It’s as safe as you place it. You can either use the jacking points on the van or place it under the suspension arms. I use a trolley jack at home, but I ain’t lugging one about “on tour”. I have 4 200x200 squares of 19mm ply in the van, they come in for a multitude of uses, and make a good base for the bottle jack. Thus fulfilling the “any object must have more than one use” rule, they make a good base for the bottle jack (a trolley jack is virtually useless on anything but a solid level surface).