With such a short length of existing (lower grade) coax, the losses may
be acceptable to a satellite system. However, switching between a
Yagi-type antenna and the dish on the roof is more complicated than
switching inside the RV, therefore I would also recommend running a
separate coax cable to the roof for the dish.

Two separate feeds from the roof; a satellite receiver box; then use a
simple A-B RF switch with 'F' connectors at the input to the TV to
select between the satellite receiver and the Yagi-type TV antenna.