Like mentioned above, you will need a direct coax connection from the dish to the receiver box. No splitters in between. The "cable in" circuit likely has numerous splitters in the walls including at the antenna amp switch.
Some rigs have a dedicated satellite coax connector on the outside that runs a direct feed to a separate coax outlet inside somewhere (usually at main tv location in living room). If yours doesn'thave this set up, you will need to run coax directly to the box bypassing all RV coax circuits. Some folks feed the wire through a slide out seal or simply install a separate coax outlet on the exterior somewhere.