You'll get a lot of opinions, and most of what you read will be subjective. What might be "struggling" to one person may be "flying up the hill" to another.
The big 3 are not the only brands than can pull a fifth wheel, but they are the only source for pickups with the 6000+ rear gross axle rating required for most fifth wheels. For example, the Toyota Tundra has a drivetrain that can easily handle a 10K fifth wheel but falls way short on being able to handle pin weight.
More subjective stuff, a diesel 2500 or 3500 pickup is NOT required to pull an 8K fifth wheel! There are several reports from owners of Ford F150s equipped with the Ecoboost engine and heavy duty payload package that are towing trailers of this size with great performance and decent mileage (9-10) towing. Also, all 3 of the heavy duty pickup gas engines, most notably the new 6.4 Ram Hemi, will easily tow 8K without struggling and get much better than the aformentioned 6 mpg.
So, again, JMO, while the diesels get better mileage and pull harder, for an 8K trailer they are not at all necessary.
