I replaced an old black & white system on my Holiday Rambler. I installed two cameras and one monitor, and documented the process.
You can download the document that describes the process from the link below:
camera/monitor installYou can save a lot of money by shopping on ebay. For example, I am using this monitor with a side view camera, and it is a good monitor for under $30.
inexpensive monitor ebayThere are many choices for cameras. Generally, the CCD types are better than the CMOS types. Unfortunately, many vendors do not clearly identify the type. I'm using a 120 degree view CCD type that works well. Consider the mounting method already used on your coach when you're selecting the camera.
If you want to do it the cheapest way, order the camera and the monitor, see what connectors you need, and try to adapt the cable that's already installed. If you're going to scrap the old camera and monitor, you could cut those connectors off the camera and monitor, and reuse with the new ones with a bit of soldering after identifying what wire does what. One consideration is that modifying the wiring connectors on the new devices will probably void any warranty on the unit.
If you're not comfortable with wiring changes, then order a complete kit with new wiring. I referenced a good vendor in the download mentioned above.
Fred
Fred & Vicki
St. Augustine, Florida