I had the same problem but was told by my RV dealer that the camera/monitor doesn't work on distances over 30 feet. I called the company that sells this product, described my problem so, without any other questions, they sent a booster antenna and about 30' of appropriate coax. I've installed the booster but have yet to hit the road but have tested it and it seems to work now. This may be your issue as well.
Jerry & Lisa Hooper
2012 Ford F350 SD King Ranch CC 4X4
2015 Dutchman Voltage 3970 toyhauler