For the most part GN cattle trailers don't pull hard.  I get the best trailering mpg with our stock trailer, everything else is worse with my 2008 duramax.  I question if one really needs to upgrade for 5000 mi per year?  We decided to get a replacement for the 2008 a couple years ago, but we were pushing 400,000 mi on the truck at the time.  The replacement is here and the 431,000 duramax still lives on the stock trailer; when it dies the new one will go under it.  We put a tad over 20,000 on the 2008 last year and half of them were towing; i figure the miles on the 2008 cost a lot less than ones on the 2025 so I might as well keep driving it.

Edit to add:
The new diesel just goes and does not seem to care what is behind it.  I have not driven an L8T but the numbers look comparable to the LD7 duramax.  After driving diesels, it is nice to not have to listen to the engine scream to make power.  My understanding is the gas is a disappointment if you try drive it like a diesel; you have to be willing to wind it up and let it scream to make the power.  With that said, if you want a diesel buy a diesel, if you want a gas buy a gas.  People will try to make you justify the price tag when you buy a diesel, but never seem to care when someone chooses a High Country over an LT trim and claimes you get the added money back when you trade and it is a similar price bump.