A good vendor for rear view camera's, and one that gives a lot of support to our RV community, is RVCAMS. I've never used them, but I've seen a lot of complimentary posts about their customer service.
link to rvcamsThere is good quality merchandise from this vendor on ebay. I've been using one of his monitors and camera's for several years. Camera has screw together connectors to the cable for better weather resistance.
link to Leviathan Holdings LLC store on ebayAs far as the wired vs wireless question, I've only used wired so I have no experience with the wireless.
I've added dedicated left and right side cameras and monitors to my coach using some of the cheapest that I could find on ebay, and so far with good results.
When you're looking at cameras, generally a CCD type camera is better than a CMOS camera.
As far as how many, it's all personal preference. I've experimented with the cameras a fair amount. I have two on the rear, one for close up (see the tow bar and front of toad) and one for distance view to see what's behind me on the road. You can also install an adjustable camera on the rear that you can control the up/down motion from the driver's seat. I've seen one vendor on ebay that has two cameras molded into one housing for close up & distant view.
I tried side view cameras tied to the one monitor, and did not like it. I finally added dedicated monitors to the side view cameras and really like this setup. Each side view monitor is in my line of sight when I look at the side mirror. So in my case, 4 cameras and 3 monitors. The always on side view camera/monitor really helps in heavy city traffic, or when it is raining hard making the mirrors difficult to see.
The cost for the side view camera, cable, and monitor was about $80 on ebay.
If you want to read about how to install the wired camera/monitor, I posted my experience in a downloadable file on IRV2 at link below:
Removing Intec monitor and installing LCD monitor/2 camerasFred
Fred & Vicki
St. Augustine, Florida