#1 it is a complete fallacy that motorhome diesel engines last 500k miles - those mileages are reserved for OTR truckers who almost never switch their engines off. Motorhome engines are abused all of the time by stop and go and have similar life expectancy to a gas engine.
According to RVIA, the average diesel pusher motorhome travels less than 5000 miles a year. The average 18 wheeler diesel tractor about 2000 miles a week. That is where the difference is in expecting around 500,000 miles on a diesel. If the motorhome was on the road 2000 miles a week it would also get an expected half million miles but it sits so much and then is run for a day or two and then sits sometimes for a month or more again. How long at 5000 miles a year would it take to amass 500,000 miles? --- 100 years, an unrealistic expectation. The 18 wheeler tractor does 500,000 miles in a little over four years. Still, diesel is the only way to go if the weights are heavy. There are no readily available gas engines with 1200 or more foot pounds of torque. I am not a motorhome guy but if I was it would be a diesel pusher.