Someone else will have to chime in on the concept of conditioning the engine as I am not familiar with it. I remember hearing about it as a kid but not recently. You could ask the rental but I can't imagine they do anything like that.
One thing to know is that RVs are not truck delivered. They are brought out by delivery drivers. So unless you pick it up at the factory or live nearby the first 1000+ miles is done by the delivery driver. I suspect they drive as fast as they can get away with, straight through. If the one you like is manufactured in Elkhart Indiana then there is about 1200 highway miles on it when it arrives in Denver. (Disclaimer - I have no idea where these ones are built). Then if it sits on a sales lot there are a number of short test drives by people like my husband who like to gun the engine before being sold.
So if you are looking at a late model used you are likely looking at one where the owners made a mistake, or bought a lemon and are trying to get rid of it. They may have used it 3-4 times the first year. It may have sat in the shop the entire first year and they only got 1 or 2 trips out of it.
A rental is the exception. These folks know the problems when they come in. They check and fix all the wiring connections. They check and make sure the plumbing is hooked up properly so the unit doesn't flood. If the steps don't work they fix it. Leak in the windshield or roof, done. Misaligned slide, they can't rent it that way so it gets fixed. They are familiar with and fix any known or anticipated problems.
I wouldn't hesitate to buy a rental over a new or used late model unit because all the first year issues are worked out. It is their business to keep them on the road in tip top shape. If they don't then they lose money.
Look at the rental prices for the Thor Citation 24ss = $197 -$414/night. The $197 is only in the winter months and then you have to pay $245 for de/rewinterization. Plus $97 rental prep fee. Plus $265 value package or $465 the works package. Plus extras for pets and bikes. Plus mileage at $.39/mile over 100 miles per night. Not hard to do in the mountain west. Plus $1000 damage deposit & $250 cleaning deposit. Plus rental insurance. As you can see these are very expensive trips when all is added up.
For the most part the people renting these units are affluent, likely older, Americans and well to do Europeans on their dream vacation.
The damage likely to be done would be running the refrigerator, not level. Hooking up too many things to the electrical. Don't know if there would be hidden damage from this as it would just blow the fuse. Maybe misusing the steps by stepping on them going in and out. Maybe flushing the wrong things and gunking up the seals in the plumbing but they are easy and cheap to change out. Also you would see if they didn't seal right before buying the unit. Also possible slide abuse. Maybe sitting in it going in and out or not leveling the unit prior to operating it. I suspect that all but hidden damage to the refrigerator would easily be taken care of by them prior to selling. All of the above are the types of things many new RVers do to their own unit.
I go back to who is selling a slightly used one. It is probably a first time RVer who made a mistake. This person is just as likely to make the above mistakes as a renter.
For me I am very fearful of new units due to all the horror stories so if I were to buy a model that this place was selling as a former renteal I would see that as an advantage.
The other advantage is to try before you buy. Like you said, there isn't that many places renting the small diesels or class As. Most rentals are Cs. I think it would be great to try a type out before investing all the money to buy. I'm not trying to convince you to buy a rental or rent. I simply threw it out there as an option because so few people know about it.
Here is a link to the current listings. Some are new and some are previous rentals.
https://www.rvtrader.com/rv-dealers/...alerid=2747976