If you repair shop verified that there is a good signal through the coach, you may need to simply replace the cable you use from the exterior connector to the campground cable connection.
Use RG6 cable and keep it to 25' or less. Get a good connector to and make a second 25' cable for the rare times you need something longer. Some of the cheaper RG56 cables breakdown and start causing problems.
If the park has cable and doesn't say you need a box, then you should be able to hookup the TV and search for a signal. I haven't run into too many that have a QAM only for TV signal.