It would be the same as when you buy a car/truck. You would need to make an offer, have it accepted and they will make up a contract, you'll either get a loan or pay cash, there will be additional fees that you will need to pay and this depends on the dealership and states involved. You will also want to have this added to your insurance policy or have a separate policy set up for it.

Adding that the underbelly should not be the only concern and even if that seems like something you are OK with, consider that a refrigerator that needs to be replaced could easily cost $3,000, the rubber roof may need replaced, the A/C unit may not be cooling properly, the propane lines may be leaking and from our experience, the electrical system can become "quirky" not to mention all of the items that Lucky Mike mentioned in another post.  If the tires are original and 6 years old, they may need to be replaced even if they don't show that much wear since rubber rots.  I just don't want to see you end up unhappy so try to cover all your bases.  It is OK to get much less than perfect as long as you understand what is involved in making it perfect for you. 

No, the dealership would probably not replace the used tires but the inspector should be able to give you an idea of how worthy they are.

We have shopped the larger RV dealerships and they are trained to sell so I understand that I have been where you are. So, take it for what it is worth to you, 1. Have a price you are willing to pay for this unit or any other that they might want you to look at and stick to it but don't clue them in on what you have to spend. 2. If it seems at all practical, look online at their other units and see if you can pick out a couple of others that might interest you in case the MS turns out not to be "the one". 3. I would spend time at the dealership but then, I would take a long break, go get lunch, look around and talk the purchase through and if the salesman says it might get away, I would go with the standard "If it is met to be..........". Return after you get some air because they will use high pressure tactics most likely so take your aspirin with you. We are "bargain" shoppers and have bought some things that others would pass by but we did understand the risks and sort of what we were doing and this was with everything we buy. We are "different" in the way we do things so this might not be the way you want to go.

The RV tech should be able to give you some idea what might need to be done and a general idea of how much it will run you.

Have you decided about whether you will want an extended warranty? That might be something to consider and there are choices available and like insurance, some are good and some not, and coverage varies.

Read everything twice and you can google "making the deal on a used RV" and get a wealth of articles.  Good luck.

Yes.  That has already been covered in-depth on another thread that I had started about with what to tow a Mobile suites.