My process would be a bit different:
If it is out of state, I would hire an RV inspector and request a written report. I think the cost, depending on the level of inspection is between $300 to $1100. Before I got on the plane I would want some confirmation that the coach looks like the pictures of the coach, no smell of smoke or pets, or evidence of such, no evidence of a water leak or damage, etc. However, an RV inspector is not a chassis expert or diesel engine expert.
Local unit Or after traveling to the unit with the written report in hand, I would access a PDI checklist and go through the unit myself. Top of my list would be looking for evidence of a current or past leak. I would look under the chassis to see how much rust there is, look at the DOT date on the tires and decide to replace somewhere in the 5-7 year range regardless how they look on the outside.
If all is going well, I would hire a leak detection service to do a pressurized soap bubble test, pull sample of all fluids and send them off to the lab, take the unit to a chassis center and then take the unit to a diesel mechanic. They have test equip on hand for that and the expertise.
Confirm the unit can be transferred to me, do a VIN check to see what pops up (both as to accidents or theft), if not too old call the mfg and see what they know about the coach as to the work done on it,, call the current mechanic (with owners permission) and discuss what they have done and recommended be done with the coach.
Keep an eye open for screwy stuff. Crappy body damage repairs, make do repairs, problems that should have been fixed if the owner was on his game. Recently looked at a Newmar Ventana. The valances had been replaced in the main section, but not in the bathroom or bedroom...sketchy. Damage on the underside of some of the baggage compartment. Lots of rust underneath, 10 year old tires.
Be sure to get the repair history if available or phone access to all prior mechanics.
Do a general internet search on the year, make, and model of the coach for recalls and problems. If there is a problem with that model coach it likely was discussed in one of the forums.
Be sure to look the owner in the eyes when you ask him why he is selling the coach.
We'll that's enough info for tonight.