I have to drive when we go camping anyway because dh doesn't drive. If I could avoid it I would!
Obviously I don't know what cars you have but I would consider putting the roofbox on the lowest one. I can't put anything in our roofbox because I'm too short which is very frustrating (especially as I'm better at packing stuff into a small space than dh but I have to keep quiet in case he says "oh you do it then" ).
Is it possible to arrange insurance for your dh just for the duration of the holiday or do you go away quite often?