My refrigerator is on the opposite side as my diesel tank. My water heater is on the same side. The flashpoint for diesel isn’t very low compared to gasoline, and they don’t have a lot of vapors. I think there is more danger from my water heater, but I don’t intentionally have it on.
Most refrigerators in TCs are electric and propane not 12V. If mine was 12V, I’d consider running on 12V, but it is not.
The ferries I’ve been on with RVs tell you to turn the propane off. Keep in mind that leaving the fridge off is not the same as turning the propane off. Even if you turn off the propane, there is still some propane in the line.
Although possible, I think your timing would have to be very unlucky to have the referigerator or water heater have a flame significant enough to light gasoline or diesel while fueling. I think it’s more likely to break a propane line in the RV, but that isn’t going to be that common either.
In summary, I don’t worry about something like this that has a very small chance of happening.