Have you noticed the diff in price of diesel vs gas lately? Around her it's about $1.00 per gal.
Price a reman engine for each. Not sure on a gasser but my 7.3 is about $10K for reman long block. Yes they have a reputation for running much longer unless a catastrophic event takes place. That said I will add that I am not one to trade trucks every 2/3 yrs. I look @ long term.
Like I said I love my truck but If I did not need it I wouldn't own one. If & when I quit camping I will sell it. Have never seen anyone run oil 15K between changes unless they are not sending it to a lab. Blackstone suggested I only run mine 5K with the towing in extreme heat here in Az. $100.00 for me to do it @ home.