In the united states, the law doesnt care EXCEPT when crossing a water reservoir dam, or when riding a ferry, or when fueling within 50 ft of a gas pump then, by law, its supposed to be shut off...
(diesel FUMES do not become explosive until the diesel fuel ITSELF reaches a bit over 100 degrees, where it then starts to emit fumes.. but still, the correct fuel air ratio must be met before there is any chance of an explosion)....
with the propane on and the fridge flame running, its theoretically possible, but highly unlikely that there ever would be the right concentration of gas pump fumes reaching the control flame of the fridge.... as the air inlet to the back of the fridge is normally many feet away from the pump (the law deems an open flame is reasonably safe if over 50ft from a gas pump).... and the flame is enclosed behind a trim shield, and then enclosed again behind the wind/flame protector shield of the fridge unit....
the flame doesnt operate ALL the time, but only when the thermostat calls for cooling, so its possible that it wont even be on at the time of fueling... in addition, the way the system is designed with its designated upper vent/heat escape at the roof, and the openings in the trim cover below, any concentration of fumes have the best chance of being passed up thru the venting system without ever reaching the flame... or will be diluted with more fresh air (with a good chance of making it no longer volatile) as it makes its way around all the shielding to the flame......
personally, I feel its safer in every respect to be traveling down the highway with the propane on, than it is to have it on when I go to sleep at night when im IN the trailer..... and EVERYONE leaves it on at night.... so my thought process causes me to wonder, are the safety concerns about leaving the gas valve on driven by the dread of possibly of WATCHING the trailer go up in flames, or the dread of possibly being killed by it?..... the few laws that are in place are mostly for the sake of protecting other people...
so, with all that said, and after nearly 50 years of staying in travel trailers without an incident, im one of those who leave it on ALL the time except when its in storage... or when the law says its gotta be shut off...
but I have SEEN incidents and accidents and 1 explosion, but its ALWAYS been an "operator error" that caused it, and NOT simply because the propane valve was left on....