Simple solution would be to buy a refrierator conversion kit and install it on a 55 gal drum (plastic or steel your choice but plastic won't rust) then make a bracket on the trailer to hold the drum. Make sure to add a valve on the bottom of the drum to drain water off as the ice melts. You will also want to add some sort of bracket beside the drum backet to hold the CO2 bottle. Make sure to tell your customer to be carefull when putting kegs in or taking them out so as not to damage the hoses or faucet hardware. Even easier but a bit more expensive is to buy a MicroMatic Party cart and make a ramp and holding bracket for it on the trailer, this will also give your customer the ability to move the keg away from the cooker once he gets to his location. Heres a link to the party cart:
http://www.micromatic.com/draft-keg-...d-JEPK-DA.html
Semper Fi,
The Gunns
"Beer is proof that god loves us and wants us to be happy" - Benjamin Franklin