I think you have a 220v three wire run to hots and a ground,
No neutral
No self respective electrician
Should have run a drop from the main service, with out a ground wire
If that is a neutral, the previous owner of the property did that run with out a permit
Sounds like you just recently acquired the property, and you want to use that for an RV 50 amp service
RVs set on steel frames and rubber tires , you want that ground for safety
As to whether you can drive a ground rod and ground that way or whether you must run a for grounds wire service
Distance from the service panel and pole, will be decided by local code,
If it's in conduit, a forth wire for ground can be pulled in
If it's direct burial cable them You need new cable if code requires the forth wire
You must have some kind of ground for safety
There is a ground rod now
But are things up to code ?
We don't know