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.