Doubt that a schematic would help even if there is one. These things are put together on an assembly and I can't tell you how many times I've found wires done improperly on the various RV's we've owned - especially audio equipment.
Your best bet is look up the specs on the head unit itself and understand the connections there and then to the splitter. Then you can correct or re-wire the way it's supposed to be and you'll know how it functions.
Pretty common for RV's owners to have to go back and correct things from the factory with little in the way of documentation to help. Nature of the beast.