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.
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.
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 are red flags:
VIN checks are not perfect but can often 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.
Phybere, discussing Ford vans, says this about rust:
“The spot to check on these is on the rear suspension, where the leaf springs attach to the body (the shackles). If something rusts off, that's gonna be first. There will be rust, but in general it's fine if the rusted thing isn't rotten/flaking apart.”3)
If the vehicle still interests you after the above it is time for a pre-sale inspection.
The potential camper 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 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 ; the mech will be able to ballpark the costs of the needed repairs.
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.