I think we might have a difficult argument on our hands if we attempt to get the local fire marshal (who is the enforcing agency for NFPA requirements) to fine anyone for filling on the vehicle, based upon Seigal gas company's how to fill a propane cylinder from a dispenser tank. doncha think?
I notice that while Joseph offers up the Government regulations, but doesn't specify where in those regulations the OP questions get answered.

I would also note that, in truth, you should NOT use the "spit valve" to offer up a guess or to decide you are "80% full" but rather use a scale and fill to weight and actually know when it is 80% full.

Both of those are items that are generally considered fact, (80% full when spitting runs thick/ fill only off vehicle) but just are not backed up by engineering.
Rather than suggest they remove the tank, how about insuring the park brake is set and works correctly and engine turned off, since the only real advantage would be to insure the tank remains stationary while filling.
BTW, I would be very pleased should someone be able to point out the particular regulations that specify the tank must be removed from the vehicle, from NFPA, ITSDF, or OSHA.