You'd be fine if you used two ratchet straps, one on the front and one on the back cross bar, then used some medium duty nylon rope, 1/4 inch would be more than enough, and tie the front down to a point under the front bumper, using a small dish towel between the paint and the rope to protect it where it will touch the bumper. Then do the same in the back down to the hitch. This will stabilize the ends so that the short distance between the crossbars will not be a problem, as the ends cannot move due to the downward tension from the ropes.
Make sure to use some foam or some small rags folded up between the ladder frame and the crossbars too so it doesnt scratch the paint on the crossbars.
I HIGHLY recommend you DO NOT place the ladder in the passenger cabin. If you got into a wreck for whatever reason, there's no telling where that thing would go, and it all would depend on where you got hit. Being 16 feet long, 70 lbs, in a wreck, that thing has a ton of leverage and potential engergy.
Really really really not a smart idea.