I would just pick up a proper 50' 30A RV extension cord and run it through the grass. At the track I spend a lot of my time at this is common that power is at the end of the paddock row. In all my years I have not seen cords melted or other issues. At the same place I have to run 2 30' 50A extensions ontop of the the main cord to reach power. I have never had voltage or connection heat issues either even in mid summer with no shade on the camper.
But I would say that for longer term, ontop of running the 30A for the trailer, I would get a 75' heavy gauge extension cord and plug into a seperate 20A breaker. This would give you extra power if needed for items like heat tape for water hoses, general outside lighting, etc to avoid drawing too much off the RV and popping a breaker.