I had a similar problem with a diesel pusher. Someone told me to open the tank cap on the other side of he RV to release the pressure. That was a big mistake. After the tank got to a certain level, the fuel puked out of the other filler.
After that I always filled the rig at less than full volume. Try holding the dispenser lever part way, as OldArmyJeep suggested. But by now, you've probably figured out how to fuel up without a mess or without particularly filling the tank.
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