Overbrook-Bill nailed it. We owned two Casitas and traveled over many states pulling them on all types of road conditions and elevations.
A V-6 engine is marginal and most will struggle on hills and long grades. Been there and done that with two fully loaded Casitas over the years.
The best tow vehicle for a 17' fully loaded Casita with a 450 lb tongue weight is a full size, four-door half ton pickup with a medium size V-8 engine and towing package, IMO.
Take your pick. Ford, Chevy, Dodge or Toyota. Drive them all and pick the one you like best. It will be with you for a long time and you will spend a lot of time in the seat towing if you camp a lot.
Mileage will be about the same for a V-8 truck towing a Casita as a V-6 that is struggling. As a rule, you will probably get about 12-13 MPG towing a Casita at 60 MPH with either.
But you will have much better power with a V-8, not to mention more safety due to larger brakes, tires, and suspension components on a truck that are engineered for towing.
Best of luck with your choices.