I think that while you will find the engine must be turned off, vehicle properly parked (park brake on, forks down, and not near an area where heavier than air LPG vapors may accumulate such as floor drains or elevator shafts) and all debris and possible sparks must be removed from the refueling area, that there is no requirement to remove the tank from the vehicle, any more than there is a requirement to remove the fuel tank from a gasoline powered vehicle.
There is a requirement to turn off the fuel at the tank and to run the remaining fuel out of the pressurized line on the tank prior to disconnection from the tank.
you must also wear face protection, long sleeves and "gauntlet" gloves.