We too have a 2004 Monaco Diplomat 40 PAQ. bought in 2014 w/40K miles. Now has 50K. These were quality coaches. Others can check my history on this, but In about 2000, or so, Monaco purchased the Holiday Rambler line from Harley Davidson. Compatible product lines & similar quality, including that both utilized the Roadmaster chassis, airbag suspension & Cummins/Allison drive train. Monaco merged the two lines over a period of time. Our diplomat PAQ floor plan isn't shown in the Monaco archive, but it is in the Holiday Rambler. a friend of ours has an identical floor plan in an Endeavor. I believe that ours is the first year that Monaco offered 4-slide floor plans. So in all likelihood, ours is one of those produced as they completed the merger of their offerings. I consider the quality of the two to be much better than most of the stuff you can buy today. Fleetwood purchased Monaco in about 2012??? Not sure what has happened to the brand since then. We love our unit. With only 35K miles, the one you're looking at is barely broken in.
Tires, Battery's and Maint records are always a good thing to look at. This rig also probably has a Norcold 1200 series refrigerator that should have a recall installed. I would verify that it has been. It can be viewed by removing the outside panel. there shoud be a small black module mounted on the cooling coil with a red light --- That isn't lit!!
Good luck. Enjoy!