And last but not least something that You can put a million miles on and be still going strong.
At 2000 miles a week an HDT over the road might last a million miles. At 15,000 miles a year it will not. At 15,000 miles a year the truck would take 66 years to amass one million miles. There are a growing number of Cummins/Rams that have passed the 500,000 mile mark and a few with over one million miles on them. But, those are rare and are on the road much of the time. So if you found a HDT with 500,000 miles on it you still would need 33 years to get to the second 500,000 miles and the engine probably would not make it being driven less frequently as most RV's are.