welcome to CPF...
I dont know about lights on a truck, but one thing's for sure...switch everything to LED and preferrably add more batteries under the back seat...
Unfortunately I do not have adequate expertise to assist you in your project, for that I apologize for the lack of education in my field.
-If you are planning to park this car in sunny places, try investing into photovoltiacs and add extra insulation on the battery heat is a no no when your trying to hold a charge.
ex:
http://www.solar-panels-for-energy.c...S_weekend.html
-If you want to conserve amperage on the cars main circuits...try investing the interior lights or side lights with LEDLAs...short for Light Emitting diode lamp assemblies
ex:
http://www.superbrightleds.com/1157.htm
-LEDLAs cost alot more than a conventional bulb, and slightly dimmer in output of a different color temperature, but it is theretically undustructiable and should last for the cars life.
-What kind of expedition car are you working on?
I hope this is of help cuz this is about all I know on "RV engineering of electrical self-sufficiency"
Best of luck, Regards.