I do not know the length of your run but you can buy 6-2 direct bury for 2.99/foot roughly. I am sure with some shopping you can do better. A ground is normally 1/2 the size of the powers and a 6ga would give you a little extra load capacity on the ground circuit. Just bury it separately from your current run and then tape the unused wires back inside the main panel and the tombstone/sub-panel. These 2 extra wires will give you the ability to add a light or other power source near the RV panel. Remember that a little white phasing tape will allow a red or black wire to be used as a neutral.