I'll go against the grain and say that 5500lbs trailer on a 12" solid extension coming off a 2.5" class v receiver on a newer truck that is nowhere close to maxed out will be just fine.
Qualifying that with I've never towed horses which can be a pretty big live load.
Not suggesting others do it, but for reference sake I towed about the same size trailer on a 12" extension, 4klb plus camper in the bed, 3950lb rated tires on factory dodge alloys all the way to Alaska. Think that was a decent torture test of the hitch setup given the days of frost heaved road travel.
I've done the same with my boat in tow a couple times on shorter trips. About the same weight.
The RIGHT and conservative way is get the super truss hitch etc to remove the pucker factor of a simple hitch extension and a larger trailer though.
With a camper that doesnt drop the rear overhang down below the level of the truck bed, it's easier to get away with shorter hitch extensions that let the trailer tongue ride somewhere under the back of the camper. With my setup and square nose trailers, the almost 2.5' camper overhang doesn't allow me to jack knife the trailer hard without getting into the back of the camper. For this, I carry another extension I can slip in if necessary to make a tight turn if needed.
Again, this is not ideal, just giving an idea of what's actually doable vs what's ideal.