You should go into every potential buying inspection knowing more about that specific model of vehicle than the seller does. Before you even go to look at a vehicle in person, you want to know things like:
Not all miles are created equal. “City” driving characterized by frequent short trips and start-stop driving causes immensely more wear and tear per mile than cruising down the highway.
As a HUGE over simplification for the chassis/engine/body itself:
If the vehicle still interests you after the above it is time for a pre-sale inspection.
The potential vehicle is taken to a preselected mechanic (who knows you're coming!) to have it professionally inspected. This usually takes less than an hour and costs $50 - $100.
If you're buying a factory-made RV, you'll want to take it to an RV specific repair shop to have it inspected by someone who specializes in RVs. These inspections take much longer and are more expensive, because they are checking many more systems. Expect to pay $150-200 and it might take a few hours.
If the seller balks at the inspection you might sweeten the deal by offering to provide him with a copy of the inspection. This will reassure him you are honest and give him something to show other buyers if you decide to pass. If the seller still won't allow the vehicle to be inspected it may be safest to assume they are hiding something about the vehicle's condition.
On new or nearly-new vehicles the mechanic may find nothing interesting. On older vehicles there will generally be work needed immediately or at some future time; this isn't necessarily a problem, but you should take it into account when negotiating the price of the vehicle. The mechanic should be able to ballpark the costs of the needed repairs.
A clear title in the seller's name is mandatory. Most people will want a normal title. Some with mechanical skills and a sense of adventure will accept a SALVAGE or LEMON title. Regardless, it has to be in the seller's name.
For those in California, there is a minor chance that a vehicle from out of state may not be able to be legally registered in California due to California-specific emission controls. However, this hasn't been a major problem since the early 90s due to most manufacturer's making their vehicles CARB (California Air Resources Board) complaint. However it is worth researching if you do plan on registering the vehicle in California.
Before you talk to the seller learn what the vehicle's title should look like. This will vary by state. In Texas, for example, normal titles are blue. Salvage titles may be red, green, or have a red SALVAGE stamp (“brand”) on them.
Some other types of not-ideal title types can be “Lemon”, “Rebuilt”, “Flood”, “Theft Recovery”, etc. Some states lump these all into the single type of “Salvage”, while some states have individual designations.
Also check your own state's laws regarding registration of out of state Salvage titles. Some states allow registration of Salvage titles to individuals; some only allow it to licensed repair shops who have to certify that they've fixed the vehicle and can therefor issue a “Rebuilt” title.
(Interesting note: Some states don't do Salvage titles at all. Nefarious sellers may be able to register a Salvage vehicle in a state that doesn't do branding, obtain a clean title from that state, then use that clean title to re-title the vehicle back in their local state with a clean slate. This is called “Title Washing” and it is one of the reasons you want to have a VIN check done.)
If the answer to any of these questions is seems shady, complicated, or uninformed it may be wise to pass.
Tales of woe like these about the title are red flags:
VIN checks are not perfect but can provide usable information about a potential vehicle. Individual checks can be expensive, so if you are in heavy hunting mode it can be much cheaper to get a bulk allotment of VIN checks or an unlimited number in a certain period.
The most popular VIN check service is Carfax.
Next in line is Autocheck.
“Interior cabinetry is a good indication of overall construction quality. Cabinets made of paper veneer over particle board or MDF are cheap, and are indicative of the overall cost target for the construction. Rvs that are targeted for weekend use can be made inexpensively because they don't get much wear and tear.” – Gary RV_Wizard1)
The general advice on the CRVL forums is to buy on condition first.2)
A vehicle with lots of miles but in good condition with all maintenance records may be a better bet than a vehicle with fewer miles but in rougher condition or with unknown maintenance history.
Rust-free is better than rusted, but body panel rust is less serious than frame rust (for example). Your mechanic will be able to advise what is cosmetic and what is problematic.
See the page on rust for info on prevention.
All vehicle platforms have known issues; these are weak points or problems that may happen more frequently on that platform compared to others. There are no perfect vans, no perfect campers.
Knowing what the common issues are can help you look for (or ask about) common problems. Here is an attempt to collect known issues on various platforms.
If the vehicle is not local to you, consider asking a forum member who lives in that area to lay eyes on it for you. It won't be an inspection but can give you an idea whether or not to buy a bus or train ticket to see it for yourself.
There are also RV and Vehicle inspectors who will travel to the vehicle to inspect it. The price is generally higher and they will not be able to lift the vehicle onto an overhead rack for thorough underside inspection.