If you do a search you can find numerous posts from people asking the same thing.
I own a Monaco Windsor, a couple grades above the Diplomat but either way Monaco made good rigs back then.
If you are not familiar with DP's I suggest getting a inspection done.
See if there are any maintenance records, oil changes for engine, trans, gen are important.
Check all the appliances to make sure the work. Refrigerator, AC units, Water heater, furnace.
Look for any leaks at roof lines, around windows and vents. Tell tale signs are discoloration.
Check dates on tires, 7 years is the normal age out period so if they have 4 years on them you have maybe 3 years before tires. I just on a set for mine, $3K so take the age & condition into account when finalizing price.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi