Maybe the super long USB cable to the rear camera is asking for a voltage to overcome resistance which doesn't always quite get there. But this is just speculation at the moment.
I was thinking this ahead of reading the words. Indeed USB 5V does suffer loss over length and devices are not as tolerant as you might think. To illustrate, I bought a 1m aftermarket charging USB cable for my old Samsung 10.1 tablet, for the convenience of the extra length. However, it charges MUCH more slowly than the original short cable.
You can solve your problem by providing a regulated 5~6V supply switched in via a relay close to the camera. This means you can still use the USB 5V as a switching signal in parallel with the data being sent back, but supply an independent voltage close to the device (the more current you need, the more important to limit lengths*), while not loading the original 5V line.
How many amps does the camera draw? Based on this you can select a suitable regulator. Cheap yet solid solution. The two items below look like overkill but, for the price, are probably the cheapest way, combined, to provide the solution. I can sketch how you can wire the circuit up.
relay 5-12 v | eBay DC-DC Buck sent down Converter 6-24V 12V/24V to 5V 3A CAR USB Charger Module | eBay* Power is delivered over long distances at very high voltages because this results in lower currents for the same power transfer (P=IV). If you try to deliver the same power over a lower voltage, more current flows, which results in greater loss as heat.
