If i may chime in...
Even the largest deep cycle , 112Ahr, battery will only allow about 4 charges of 10S 5000mAhr packs..Sorry.
A generator is a great idea, but then you are depending on a large and usually costly 45amp 13.8v power supply to plug into the generator. HOw many of us pattern flyers go to the field with out replacement parts? Humm...i think nobody. A little too costly to keep two of everything1
I recommend a large, but not too big deep cycle battery, good quality 15 amp marine battery charger and a small generator!
I know its a lot to bring to a contest, but lets look at it first.
The honda inverter type generators are great, and cost about $1100.00. Well worth it, but now we have a large power supply that we are pushing very hard everytime we charge our packs. I have seen these large power supplys simply go POP !! without warning. Now we have no way to charge anything.
If we have a good size Deep Cycle battery (90.00 or so at Batteries Plus) with aforementioned wal mart smart charge charger ( $65.00) and the Honda generator, or a cheaper one if you want.

Now we have out battery that we can charge our packs off of of we are only going to the fieled for a flight or three. On contest day we can bring our battery and if we have 110V power at the field we can use our smart charger to keep our deep cycle battery at or near a full charge all weekend. If we dont have 110v at the field we can then breakout our generator to run our deep cycle battery charger to maintain our Deep cycle battery and fly all day and night if necessary until we run out of gas in the generator... Also we can get say 2 charges off our battery before fireing up the generator and there fore not having the generator running for the fullday during charging.

Hope this wasnt too wordy, just my two cents!