We have used our 8.5" slide in with 3/4 & 1 ton SRW pickups. Large gas (but older fuel injected) and diesel.
And a horse trailer.
There are large and lite model slide ins. A few inches here and there, less stuff, lighter and smaller. You are pulling a load and carrying a load. You will be slower.
Our then new 6000# (empty) horse trailer required the diesel (one trip loaded up and the 460 Ford was soon for sale). We live at altitude, so 25% loss on non-turbo engines.
As the camper stuck out, I first researched what my loss for a hitch extension would be. About 1/3 of capacity per foot of extension.
Bought the highest weight rated hitch I could find (16 or 18000#; don't recall, but plenty, 2.5" receiver). Made an 8" extension, all is good to go.
On the old truck, I made a dual hitch similar to what has been mentioned. It worked fine too, though that trailer was much lighter.
The towing guides from manufacturers used to mention slide in and 5th wheel frontal area limits. I suspect the marketing dept wanted to sell more trucks and the legal dept couldn't find any real reason to keep that information in there. IIRC, they also included overall side area for wind concerns. Nothing like a huge 5th wheel on a pickup to make you pucker up driving near them on the interstate in open windy country. More power is not always your friend. Same goes when driving with a big box in the bed.