I have often wondered if those would be practical from a safety point of view. I would think that the Tank needs to be securely fastened somehow.
Many Farmers have "Slip Tanks" in the bed of their pickups ,some with Electric Transfer Pumps, but as far as I know they pump into the stock tank through the normal fuel filler!
Rob & Jean
98 Dutch Star Diesel Pusher ..07 Honda CRV AWD