Just came across this on Racing Junk posted today:
2000 Freightliner FL70 For Sale
I'm going to make an educated guess and say that is an old satellite truck like the networks used. That huge flatbed is perfect to simply build your box on top of, lower tool boxes are already there. Strip out all the electrical panels and hardware and you've got the box done, just add interior. Got a huge generator already installed. Running the engine to power the generator might be a bit much for camping, but great to run tools, compressor etc. in race pits.
Hey #90, this one is right up your alley!