Michael,
As Eric notes, upgrading your complete system is an option. However first you should verify the existing system is working. If you don't have the manual, see Battery Charging Systems in Coach Assist.
One caution; if you decide to run new wiring, be aware that there are muffin fans mounted in the reefer vent (at least there was on my 00 Marquis). So, remove the roof vent cover and use a flashlight to see if there is a way to route the wires past the fans.
Good Luck, Steve