The waterproof trailer vision ones have no sound now, as microphones tend not to be waterproof. You can buy cheaper versions, but they don't have the wide angled lense
I am just about to put digital wireless ones in my lorry, you can pair them like bluetooth devices and then you get zero interference.
we bought a reversing camera for a car and put it up in the trailer , wireless cost around 50 pound from ebay dont go anywhere without it can see what she is up to peace of mind love it .