The best way to charge large batteries at the field is to use your car battery. Keep an eye on your car batt voltage and start it up every so often to keep it from getting too low. If your field has 120V outlet, then get a good 12V power supply and use it (that's what I do at a couple of the fields I go to).

This is a great power supply, I got 2 of them on sale a couple weeks ago for $35 each and free shipping! Looks like they are back up to $70, but it's still the best value per Amp output I've seen for a 12v supply:

http://www.hobbypeople.net/gallery/158301.asp