In the typical RV installation, there is no need for a splitter. There is a Distribution Switch that takes input form the Antenna Amplifier, the VCR output, a Satellite Receiver and an Aux and has outputs for TV1 (normally the living room or front of a motorhome), TV2 (normally the bedroom) and to the VCR input. The Distribution allows each of the devices to get a video signal from one of the inputs.
So yes there effectively is a splitter but it is the Distribution Switch.and the CH 3-4 Receiver output went to the Satellite Receiver input of the Switch. The Switch does not need to be removed but an additional line from CH 21-69 does need to be connected to, say. the Switch Aux input.
That does not account for why the "triplexor" is present.