I have read that if I buy a 10 year old DP with vey low mileage that may be a huge problem as diesels are not supposed to sit for long periods of time due to engine lubrication issues. Does this statement have merit?
Farmers are notorious for letting a piece of equipment sit for 10 months, then firing it up and running it hard for two months, then repeating the next year. I never heard one complain about engine problems. The biggest thing I recommend looking at on any vehicle that gas sat is soft goods like hoses and belts. You should check the brake system also, which on a diesel will be air, or at least partially air.
It's not quite so much that the engine sitting is a problem, in and of itself, but rather that a coach that hasn't been used very much over a long period (several years) may have problems in a couple of areas. One is that running an engine (gas OR diesel) for a short time, not long enough to get it thoroughly warmed through and through, will leave some moisture behind that otherwise would have been evaporated by the heat, especially in the oil, though other places too. So if this has repeatedly been the case there may be internal damage in the engine, or perhaps in the alternator or other engine accessories. And if it hasn't received at least annual oil changes and, perhaps, certain other maintenance, there could be other problems.
Another concern is aging (this is not JUST for low mileage, but that makes it worse) of things like rubber parts that when exercised will last longer.
So low mileage is an alert, but not a guarantee of serious problems. Note that the average RV is used perhaps 6,000 miles a year, so that may give you a starting point. Mine, for example, has about 13,000 miles over two years, and sometimes sits for three plus months at a time, but then gets run for at least a few hours at a time during a long weekend to two or more weeks, even on the closest trips, then may sit another month or three, but it gets its annual service, plus attention to anything else needed.
It's real common on motorhomes to have low mileage. They sit a lot. I dont' think I have ever heard of anyone having engine issues as a result unless maybe on the extreme side.
I did look at a 2011 that was highlighted as having only 9,000 miles. The Dealer grumbled when I said that wasn't necessarily a good thing, but he did come back with a good answer. To a Dealer, or even the general public, mileage is still viewed as an indicator of wear and that effects price, regardless of how technical you want to get with care of a diesel and extended mileage. 50,000 miles is typically when the bandwidth of buyers trims drastically.
I think Larry summed it up well. There is a fairly good possibility that a very low mileage rig has been neglected in terms of maintenance, whether through neglect, ignorance or simply because the owner was dealing with some other priority in life (work, illness, whatever). Some owners are always conscientious about maintenance, while others are more inclined to ignore things that are not an active part of their daily life.
This begs the question of what is "low". Most Rvs don't accumulate a lot of miles anyway. 6000 miles/year is common, even among fulltime/longtime travelers and snowbirds. Weekend campers may be more like 3000 miles/year.