We used to start off our chicks and ducklings inside our greenhouse, which is inside the polytunnel. A huge fat rat dug underneath and killed most of a brood. So we put down tiny mesh wire netting on the floor of the greenhouse - another rat, or family of rats, managed to pull chicks through this by one leg. The terriers finished off the rat, but we learnt that the floor is just as much a route for attack as the walls, windows, pop hole etc.
I know your experience is of grown hens, so maybe not a rat - I would say stoat - you are very unlikely to see those but they are around and unbelievably strong for their size.
You need to turn the caravan into a fortress
