The satellite dish tuner probably has S-Video and composite video outputs, but those will have a ground connection. But it probably also has an RF outputs that is modulated to a standard television broadcast frequency, like channel 3. That output can be isolated using RF transformers.

If you can't isolate the system after the tuner, then you will have to solve the grounding problem another way. You might want to connect earth ground for the dish at the same point as earth ground connection of your AC outlets. If you're lucky, they come into the house fairly close to one another.