2004 is one of the years where Ford V8 and V10 spark plugs are two piece. I'd have a dealership or mechanic who knows how to use a Lisle tool to coax the spark plugs out and have good, one piece ones put in. Time-serts might be a necessity as well (and the reason I mention these by name is that they seat well, don't have a metal piece that can break off into the cylinder, and don't seem to have many horror stories.)
I'd have the spark plugs, and perhaps the coils as well changed. Not cheap, but cheaper than a rebuild.
After that, what fordsooperduty said.
Once the rig is in good mechanical shape, I'd start on individual components. The water tanks, I'd thoroughly sanitize. Black and grey tanks, I'd also sanitize and clean out.
The generator can be a wild card. Some people are good at keeping it maintained. Others might mention that they put 3-4 hours on it total (which is worse than running it for thousands of hours), and this likely means that one will have to replace the carb on it, and even then, there may be issues with corrosion on windings and slip rings.