OK. That box is for picking up digital broadcast signals and converting them to a composite video signal that feeds into each of the 2 monitors. When you say you have it hooked to the motorhome cable system, is that hooked to the antenna? Is there a booster button somewhere on a plate with an antenna jack on it that changes the system from cable TV input to the antenna signal? Little light will come on.Does the original TV that came with the motorhome work? You may want to try hooking some kind of antenna directly to the Boss Tuner and then the tuner directly to a monitor. That way you can rule out the wiring of the motorhome and see exactly how well this thing works.
BTW, that box will not work with a cable TV type signal.