I would think a 38 foot toy hauler is way heavier than what I am towing and my mileage would be 10-12 mpg. I googled real world mileage as suggested and most claim 10-12 in similar situations as mine.
I have had the trailer booked up for 4800 of the 10000 miles according to my dash trip odometer so ya, it is used enough to warrant the diesel in my mind.
Don't see the 11 grand premium you say for it. Those mods you had to to use yours are not free and the additional resale I'll get help make up for what I paid to have it.
Last truck lasted 12 years and 250000 miles but reliability was getting to be a concern so had to step up. Did not want to but had to be done.