I can think of a few ideas that are designed for rough automotive use off the existing electrical system. Source an alternator meant for an ambulance or other vehicle with heavy electrical needs that runs on the same chassis (or at least uses the same engine) as your truck. There are also a number of aftermarket high-amp alternators designed to run a welder off your engine, you might need a custom bracket to mount it but they're an option and are designed to put out at least 80% of their rated current at engine idle.
Run a couple of deep-cycle batteries isolated from the starting battery so that you can drive the truck away after the sign lights have drained the lighting batteries while parked. There are available battery boxes for various commercial rigs that hang off the side of a framerail to hold the large commercial-type deep-cycles.
For lighting, take a look at the "off-road" type floodlights that you tend to see mounted on the front of jeeps, jacked-up 4x4's, rallye cars, etc. They put out quite a lot of light and are available in many different beam patterns, plus they are ruggedized and meant for the heavy vibration a moving vehicle puts on them. You might could do with some ordinary rectangular low-beam headlights, at least you could replace them easily. In either case, run heavy-guage wire to the lights and control them with relays so you don't have all that current coming through the passenger cab. I've experienced short-circuits under the dash melting solder joints and dripping it onto my foot - not fun!
For extended use while parked, a small generator to power the sign might be in order, it will use less fuel than idling the truck engine and you can put a big muffler on it to quiet it down. If you go the generator route, you can use 120V or 240V lighting, and from there you can ask the regular electricians here about suitable ruggedized lighting. Advantage to this route is that if you set it up right, you can plug it in if a customer has a high-enough capacity socket and they want the truck parked there a couple of days.
[This message has been edited by cajun (edited 08-09-2002).]