Not sure how they work, but here's my dumbed-down idea: I'm assuming you'd have two receivers that plug into the separate pairs of AV cables for the HU. Those would have to be powered in some way...guessing tapped into a 12v source. Both RV cameras would need to be tapped into a reverse signal source so that they turn on when they need to. Does your trailer have backup/reverse lights? If so, there it goes. When you need camera 1, press AV1. Camera 2? Select AV2.
This sounds like it could work...no reason not to, right? What's missing here?