If you google "adth monitor" you will find some info. RV cams sells a replacement monitor and camera.
You probably need a spare monitor and camera to be able to troubleshoot. I think rv cams does some repairs and may be able to help you with troubleshooting.
RV Cams phone is (877) 378-2267, and web site is
rv cams web siteI've never dealt with rvcams, but they have a good reputation on the rv forums.
You can buy a new monitor and camera off amazon or ebay for less money than having the old one repaired if you're handy and can splice into the old cable. Even buying a new cable is not hard to run, just have to drop down rear cap, under the rv, and back up into the cab area.
Fred
Fred & Vicki
St. Augustine, Florida