The dish antenna must be directly connected to the receiver. If there is anything device between the two such as a RF splitter or AV switch matrix the set up probably will not work. The coax provides two way data. Pointing commands and such from the receiver and the down link TV signal and such from the antenna. You might get lucky with an A/B switch but I was never sucessful.
What I do when the roof antenna is blocked (last owner installed) and I need to use the carry out I'll plug into the park cable outlet on the side of the coach but I'll also disconnect the coax for park cable from the AV switch matrix and connect it directly to the satellite receiver in place of the coax from the roof antenna. Actually I've never used park cable since I have a satellite subscription.