Lots of "it depends" to your questions.
First, if your outside coax connection is labled "cable" then it probably goes to or through the powered switch plate (for switching between your roof mounted over the air tv antenna and cable) and then through a cheap coax splitter behind the panel the switchplate is mounted, and going to each tv. You cannot send the digital signal from a portable satelite dish through that cable, switch or splitters.
Second, IF the outside coax connection is labeled "Satellite" then it should be a single coax cable making a direct run to your equipment cabinet going to a similarly lable plate connection or loose cable. If so then you can use this for a portable satelite dish, but you will only have a single coax which may only be good enough to feed a single, non-dvr receiver.
As for getting the satellite receiver output to all 3 TVs -
1) IF you have HDMI cables running from each TV to the receiver cabinet, you can get a HDMI splitter or matrix (i.e. 2x4 matrix so you can switch between a DVD player or Sat).
2) or you can get a HDMI Wireless repeater (Best Buy carries a few with HDMI pass through for the main TV and two receivers for each remote TV), if you also have a WiFi system in your RV, some people experience interference issues with the wireless HDMI, which is why I prefer the matrix switch option.
3) if you have RF remotes for your Sat receiver, you will be able to operate the receiver (change channels) from the remote TVS but if not you will need to make sure that the HDMI matrix or the Wireless HDMI repeaters you buy have an IR repeater/extender capability, otherwise you will have to be withing line of site of the receivers (or any other source like a DVD).
Hopefully this helps and isnt just TMI...