Newmar NB: Welcome to the forum and your first Newmar!
You didn't state your overall RV experience level, so I'll apologize in advance for posting perhaps too many details.
The process my wife and I use is that we arrange with the seller (we have only purchased from dealers, but would be the same if private) to have at least a 1/2 a day to literally crawl through every single item on the coach. She takes the inside and I take the outside.
On the inside, she is testing function of everything. We even bring a DVD to test the Entertainment system functions. All electronics. All appliances. Refrig should be running to show it is cold. Drawer and cabinet functions. Overall condition. Any signs of water leakage anywhere? Look INSIDE the cabinets that are against the ceiling. Go to floor level and feel around the walls at that level as much as you can...any soft spots? We want water in the tank and we test the pump and all faucetts/toilets. AC, heat. Are all the options claimed actually there and working (dealers are good ones to have a list that may not be completely accurate). Lights working? Slideout function (we do that together so I'm not under the coach). Fogged dual pane windows?
I bring coveralls and go under end-to-end. I'm looking for leaks, grease points that look like they've never been touched, etc. Tire date codes...how old are they? RV tires generally don't wear out, they age out. A 2014, perhaps built in 2013 would have tires ready to be replaced if not done so already. BIG cost. I check in all the bays for anything broken, excessive wear, water damage, etc. I check the genset function. I like to have oil and coolant analysis for the coach engine as well as the generator engine. Samples can be taken and sent to a lab. I'm checking to see if the engine compartment is clean and cared for. With the engine running I'm listening to anything unusual. Do all the exterior lights function (Headlights, running lights, turn signals, backup lights). Awning function (main awnings and windor/door awnings). Sealant condition on all the sidewall items (windows, vents, along the separation between the coach and the basement, etc. Leveling jack operation. I get on the roof and do a complete inspection of sealants. You'll know when you get on the roof if the coach was cared for top to bottom. Uncared for roofs are pretty obvious with filth buildup, debris, cracked sealants, etc.
Of course there is the test drive.
I'm sure I missed things, but you get the jist. We try to take nothing for granted and check everything we can see.
In addition, because of reviewing hundreds of posts here, I also checked these things when I purchased my current 2013 Newmar two years ago:
1. Slide out bolts and brackets.
2. Lubricant level in the engine fan gearbox....the prior owner had not installed the breather tube extention "fix" and on my coach the lube level was quite low. I took a chance on it since it wasn't leaking and sounded good, but I drained and replaced the old lube (75W 90 Synthetic gear lube) first thing and all has been fine.
3. While you are back by the fan cage, and if a Freightliner, what kind of support system does it have? Is it just a couple of metal "tubes" attached or does it have a more robust structure holding it on. I cannot remember when the problem with that occurred, but at some point freightliner "cheapened" the support structure for the engine fan and they were breaking and causing material damage when fan parts flew around.
4. Ours has the Girard Awnings and, as I had read on here can happen, when retracted the front of the forward awning was sticking out about 1 1/2" while the back was completely tucked in. It's just an adjustment, so not a big deal, but something you don't want to run around with for a long time.
5. READ on this forum and make a list of specific items. One more I can't remember all the details on (model, year, or chassis mfg) was front axle overload.
I may have missed some items but I can state this...many of the items I had read about that were applicable were present to some degree on the coach I bought. They weren't at critical stage yet, but it allowed me to assess and quickly take action on them. Kudos to everyone here that helps all of us understand and be aware of these various issues.
If you don't feel you can do this kind of inspection, then hire a competent RV inspector to do it for you. These a big complex machines with lots that can go wrong, especially if not maintained properly. It's not something to be afraid of, but certainly something to verify and try to know about in advance. If things are found it give fodder for your price negotiation.
Again, not a comprehensive list, but hope it helps.
"Second star to the right and straight on 'til morning"
On the Mississippi in SE Minnesota
2013 Newmar Mountain Aire 4336
2016 Chevy Silverado 4x4