Forget the 12 volt Xmas lights, they just aren't worth it. A normal string of cheap lights is about $3.00 and the 12 volt version will be several times that cost. Wiring will become such a hassle that you simply won't want to do it.
You could use an inverter, and there are portable inverters available up to about 1000 watts. Be careful, though, as a 1000 watts will draw ~83 amps, and at idle speeds your alternator is probably putting out less than 20 amps, so your battery will go dead in in 30 to 45 minutes even with the engine running.
Generators capapble of putting out several thouasand watts are easy to come by. Rent one if you can't borrow one. On a truck it won't be any problem at all, plus then you will be able to run the DJ stereo system and crank the Christmas music loud enough to drown out the generator noise. Just be careful that people riding on the "float" aren't breathing the generator fumes.
I have done a number of lighted boat parades with my Carver. I would keep adding lights until I couldn't keep the breakers from tripping! I had a LOT of lights on that boat! Note that you shouldn't put more than 4 strings in series, otherwise the end string runs dim and the first string might blow its fuse. Plan your lights out in advance, and run a web of heavy duty extension cords first.
Give some thought to the driver. While you are creeping through the parade the lights aren't going to be a problem. But getting from the staging area to the start of the parade and then back at the end there might be some distance of travel where it might be best if the driver could see where he was going without being blinded by his own lights. I always had the forward lights on a separate switch so I could turn them off when I needed to see.
