Value is very subjective, but could be anywhere from about $1k to twice that. In some markets, it might sell for $3-4k as an overlanding project trailer.. Also depends on what you mean by a "complete" generator trailer.
M116A3 is just a flatbed trailer. If there is anything remaining from the generators on it, or if the generators are still there, it can change the value significantly, or might not change it at all. As I said, value is subjective.
The paperwork, and not being titled, does drop the value a bit. Or rather, if it was legally titled/licensed, to some people that raises the value by several hundred dollars just not having to deal with the paperwork related issues for it.