The Folks who buy high mileage ex-rentals, are usually pretty sure, them will run 300K without much problem.
Those of us who bought newer are usually not so sure about that.
Truth is probably somewhere in the middle.
I think one thing to consider is, how long do you intend to keep the RV. I for instance will keep our's for at least 20 years and maybe more. I estimate we will put 150K on it. So I did not want to start with one that already had 150K.
But at 5K per year, ex-rental with 100K on it, in ten years still only up to 150K ( not all that much), So I think each person's expectations, plans and use patterns are different, and that should influence their choice ( as to if they buy a higher mileage ex-rental, or a newer lower mileage unit), in my opinion anyway