Onan Diesel Generator problem

I have a 2005 phaeton and had gen problems, but not really with the gen. First the filter that Tiffin installs before the genset is large and air can build in it over time if it is not mounted so that the inlet faces the ground and the outlet is facing upwards. I had this issue and it took forever to diagnose. Not sure thats what you have going but you need to eliminate it as a possibility.
Here is another tip. I unit that old is going to have build up and corrosion around the temp sending unit. Change that sending unit now while you are servicing it. Its cheap, and it will save you some hot night you need AC.