What does the panel wiring diagram show you. Are the outlets run off of the same leg or separate legs? I can not find a diagram for a 30/20 RV panel but I would guess that the outlets run off the same leg. If this is the case, up the single supply breaker and the necessary 2 wire direct bury cable size to run both outlets at the same time. If they are off of separate legs I would run separate supply breakers and the necessary 3 wire direct bury cable. This separates the outlets and will not kill power to both RVs if one has an electrical issue.
Randy