On motorhomes, they put them pretty far forward on the coach.
TerryH on this forum has installed side cameras and I think he has his mid-ship.
Where you end up putting yours needs to be a function of how far up the coach you want to see things on your sides, balanced with "where/how am I going to run these wires?".
If you locate the cameras on the sidewall where the wires come into the backside of cabinets, then hiding the wires on the outside is all but figured out.
If you locate the cameras on the side walls and NOT on the slide walls, your wiring routing becomes way easier.
Then of course, get the wires up to the gen cabinet area and out to the pinbox to wire into a receptacle, then use a coiled umbilical to connect to the truck. Unless you're going wireless - which I would not recommend unless it's a proven system (like Voyager).