I would also add, due load shedding, power consumption will vary a lot on the ground especially if you operated high current draw equipment. So an accurate steady state current draw will be difficult to determine unless you can get access the the Boeing Electrical Load Analysis information for the 747-400.
Just what I was thinking, there is likely a very significant difference between peak demand and average.
One analogy: although your house has a 100 amp service (USA 120/240 2 phase) it would be very rare to see the meter on the side of the house spinning at anywhere that rate, even with all burners and the oven going on the electric stove.
Both peak and average values would be needed to size the inverter and batteries. My (non aviation specific) gut feeling is that you would be looking at a 'lot' of battery.
If you can add a trailer hitch to a 747 maybe a Tesla electric car would be about the right size
A GPU likely has ammeters that could give you a rough idea by logging peak current for each 5 (or even 1) minute interval during a 'typical' turnaround and then summing the 'amp/minutes'.
This would provide the battery capacity (amp/hours) after efficiency and voltages are factored in.
Of course this would not be worst case but at least one data point beyond assuming maximum draw the whole time.