These should generally be placed at breathing height (not on the ceiling/floor) the instructions should have some guidance too that may vary by make.
They generally say avoid placing them within a metre of an appliance or kettle to avoid steam.
Don't place too near to a leisure battery as hydrogen can set them off too.
Very good, breathing height is the key I see. I suppose the most appropriate breathing height in the camper is the height at which you'd be sleeping.
I'll find a suitable spot for it!
Many thanks again for the information Matt.
Mine is above my head when I am asleep downstairs, on the ledge that the upper bed sits on when lowered. Alternatively, I sometimes just leave it on the bed downstairs.