On my '14 Seneca it is indeed as easy as hooking up the connection in the water/sewer compartment to the campground's outlet and then make sure the antenna booster is switched off - no green light on panel behind main TV in cabover bunk area. Then magically cable signal is available to all three TVs through the same coax connection the "air" antenna signal comes into the TVs.
Of course you must then have each TV do a "channel search" so it knows what channels are available for your viewing selection.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST