Getting good service seems to be very difficult and my thinking/experience may be way too old!
They may be correct---or not!
What I am used to seeing is the monitor has cables from each camera, left, right, and rear. My first thought might be to take the RV to a different type shop that deals in this type system, not RV specific. Somebody who deals in fixing automotive camera systems and radio?
If I understand what you are saying, it would seem to fit that the left and right cables may be reversed and one left off totally? You get no results from the one left off and get the wrong side when looking at the one you do get a picture?
Until a short time back, we had a very good interactive parts drawing setup online and we could get a really good look at things like the back of the monitor on the dash. But now that system has been replaced with one which is junk and missing vital parts of many of the systems. So bad that it is of little value at this point.
Thinking I could check for details, I found the info on the monitor to be totally useless and certainly found some info that might indicate the folks who built this new system are not the sharpest crayon in the box??
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Is there no value in having work checked before putting it out to the public?
Sorry, no longer able to give good info here but I might suggest first checking with folks who do radio/monitor installs full time to ask if cables are a likely problem. Possibly a few phone calls could get a better idea than what I have?