In your (Mike Schriber's) case, I hardly think it matters too much because you're never going to approach wearing out the engine at that pace. It's up there with concerns about rotating the tires on most motorhome when said tires are going to need replacement due to age rather than treadwear.
I think the main concern for the age limit is that putting few miles on the vehicle is usually associated with short trips where the engine doesn't get up to temperature and so many contaminants that would otherwise boil off just accumulate. That's not really the case for you (or most motorhomes, really), so the time limit is probably not very important. Oil itself doesn't have a particular shelf life after which it goes bad or sours or something, at least not in the timeframes we're talking about.
