We've taken a few long trips with our different 5th wheels. From Michigan through the Rockies and back was about 5,500 miles. Our trips to Florida and back for the past 3 winters were in the 4,000/year range. Many RVers don't take those kinds of trips ever, preferring to keep it within their own state or within a couple of hundred miles max.
As jplante4 mentioned, RVs need to be used. If you're looking for a motorhome low mileage is still an important issue, but that's for the motor only. The other mechanicals (furnace, fridge, AC, water heater, etc.) are really important too, especially if you plan to spend time living in it. Those items should be checked and be in good working order.