How to check on old DP's engine, transmission ,radiator & Chassis

Take the VIN to a Monaco dealer and see what they can tell you. Ditto, the engine S/N to that engine’s nearest dealer and have them run it also.

A diesel of that nature isn’t the kind of thing a shade tree mechanic whips out of the chassis and does serious wrenching on it himself.  

Basic fluid checks and put it through its paces. It either runs flawlessly or not. 
Then get a complete service. Change all fluids and filters and let er rip. 
But, why buy an old repo from a dealer and have zero backing from them for likely way more money than buying a private party unit that you have a better chance of knowing the history?  The market is flooded with good cheap low mile 15-20 year old DPs. 
Or if you’re set on buying this one make the dealer do a bunch of work that you can verify, like everything I said above. Along with brake inspection. And how old are the tires?  

I would not buy a diesel.

Is the engine a Cummins or Detroit?  If the latter, stop and do not pass go.  Alright, I am kidding on that one, mostly.  Removing a cover on the engine would allow you to get a general idea of the cleanliness of the engine.  It'll either be clean or not.  However, not exactly a straightforward task, and will require gaskets, etc., (as I am sure you already know).  Fluid tests aren't a terrible idea, they will let you know if there are abnormal amounts of metal, etc, that shouldn't be there (i.e. bearings).  A mechanic could give it a once over, but unless he brings a diagnostic tool, he's not going to be able to do much more than an educated and mechanically inclined buyer (he might do worse).  You could get your own scan tool, like a Silverleaf VMSPec, and take it for a spin.  As another reply stated, run it up grades, get stuck in traffic, etc., and watch temps and pressures, then see if any codes pulled. Chassis wise, it'll be a lot like a car or truck.  Drive it, does it wander, does it make bad sounds.  Pump the brakes a lot and see if the air system can recover.  Get under it and look for dry rot on the air bags and other rubber.  Look for torn boots or signs of leaks, fire, rodents.  Basic stuff, really.  One of the best indicators will be does it look well take care of?  If you are satisfied but still have concerns, make the deal with a claw back clause to protect yourself for things you might find out later.  Don't forget to look at some of the other big buck systems such as the generator and Aquahot.  Overlooking something there could cost $10k, no joke.  Good luck, some of those 2000's Monacos (early to mid) are pretty cool!