Looks like you found some nice travel trailers. I'll bet you will be happy with any of them.
I have owned a Jayco Jay Flight and a Grand Design Transcend. Both have been good quality and I would happily have either one again. Can't speak to the Kingsport.
A few thoughts.
Check out the heating ducts. Grand Design has ducts in the walls and cabinets. Jayco are in the floor. Technically, floor ducts are supposed to perform better. But, dirt and other things may drop into the ducts. Cleanup will be easier without floor ducts. In cool weather, you may find a small electric heater is sufficient to heat the space, only using the furnace for camping in very cold weather.
Be sure to look under the camper. Things may have changed, but our Jayco had a cloth covering for the underbelly. Our Transcend has a rigid plastic material. With some heat ducted into the underbelly, it's supposed to improve three-season insulation. However also be sure that the underbelly is installed with minimal gaps.
The Grand Design indicates it has a 15K AC. I don't see specs for the Jayco but it is likely 13.5K. The 15K may give you a bit more cool air, but in a small camper, either is likely sufficient. A downside we have found for the more powerful AC is that our Grand Design is set up for 50 AMP power. The heavy cord is detachable rather than contained in the camper side. Many places we camp only have 30 AMP connections which means having an adapter. You may never need a generator and most campers stay where they have hookups. For some travel planning, I've been looking at generators. A Honda 3000i says it will power a 13.5 AC but not a 15K.
It may be something you never use. But, check out the outside shower. Jayco uses a pretty standard box with a shower handle and hose. Grand Design uses a quick connect with a garden sprayer. Not sure one is better. Different approaches. The quick connect is easier to keep clean. Either way, it's something you'll have to winterize if you live in a colder region.
The Imagine will have fiberglass siding and an aluminum frame. The Jay Flight is aluminum over a wood frame. You can search the forums for past conversations. Neither is necessarily better. Many prefer the smoother fiberglass look. However, some have had stickers fade on fiberglass. If you ever accidentally bump something with your camper, the aluminum may be easier to repair.
Water hookups on the Grand Design are inside the storage compartment. They call it a universal docking station. The Jayco likely has a more traditional external water connection. Both work well. The docking station approach might be better for cooler weather. If you go that way, you might find the water connection tips on my blog helpful. One thing to watch for, on some Jayco models, the water connection or outside shower are located near or above an electrical hookup. Just be sure to review your specific model so you are comfortable with the setup.
Hope that helps. Good luck in your search.