a rely is only used when a "unit" is a high drain/amprage, such as a high power amp, off road lights or such. For a low drain unit like interior light, radio it doesn't need a relay. a fuse is a good idea from the power source to your "unit". A backup camera and a small monitor wouldn't need a relay. It wouldn't draw that much amps...from the looks of what you have in your Expy...you have a clear idea of what you are doing. Your back up system wouldn't be on for no more than a few minutes at a time if at all 60 seconds, your radio is on most of your driving time and it doesn't need a relay to keep it going.
Just my take.
take noted...