Winnebago installed a 45 amp converter in my RV with 8g wire with a run about 50% longer than yours.
That said, I upgraded to a 55 amp converter and 4g wire, but that was just my personal choice. It was overkill but reduces voltage drop and doesn't hurt anything.
ON EDIT:
I just surfed a bunch of websites that recommend wire sizes and the results seem to be:
8g - safe but significant voltage drop and poor charging performance
6g - good choice
4g - great choice because it reduces voltage drop significantly for best charging if you boondock a lot.