Actually, just attaching the ground terminal to a ground rod won't do much of anything. The ground rod would need to be bonded to the neutral back at the main service panel.
I find it very odd that's it's wired this way. What kind of wire comes to it? Is it an flat SE style cable with 2 insulated wires and then a bare wire with the strands wrapped around the 2 insulated wires?
I wouldn't use it without a ground. It would leave you susceptible to having a hot RV chassis if a wire ever shorts to the frame on your RV.
The GFI solution mentioned would work but a 50A GFI breaker is really expensive.
