Let me describe teh problem as best I can with ASCII
Car
Filler
. . .\
, , , ,\
' ' ' Tank
RV
Filler\
. . . ---__Tank
As you can see on the RV filler tube is running near flat v/s a serious drop on the car.
The result is at high volume the filler hose backs up shutting off the pump
Solutions
If you have filler caps on both sides, remove both gas caps
If not, Use the jacks to "Tilt" the RV away from the pump
Pump slower (first notch or less on a latching handle)
This is in addition to checking vent lines and other things suggested above.