4 season TT

The last post finally said it...
The roof AC will cool the camper, insulated or bad insulated. How well insulated it is will determine how long or how often the Ac unit will run or come on. I had a black class-c and it stayed a meat locker in 115 deg desert in the sun.
Big windows will be big heat losses, but you can cover them when you are out or keep the shades lowered a little to reduce air from constantly circulating over a hot window.
The roof is a big source of heating too. Some roof coatings claim to keep the roof cooler, idk if true.
If you are at CG then AC compressor running all the time not an issue, note the fan will run 100% of the time, but if on a generator then the cycling will burn more gas.

X4 on ORV or Arctic Fox. I have zero problem with my single A/C cooling my trailer in 100F weather in direct sun, and my trailer is almost 400 square feet. The slide toppers did help a lot though.

Arctic Fox and Nash, made by Northwood MFG in La Grande, Or. are considered to be 4 seasons campers as well as being well made. We have owned 2 Arctic Fox 5th wheels-the last one we put about 50,000 miles on it with trips from So Ca. to the Yukon Terr. and to Fl. any many points in between. They make their own trailer chassis which comes standard with shock absorbers, etc.

It's going to need to be one that is aluminum framed and aluminum smooth sided. REALLY hard to find.

Your requirements will be tuff to meet. As wizard said under 20 is danger time regardless of the 4 season sticker...28 sustained IMO.

My Outdoors RV 18RBS is 5400 pounds dry weight, so finding a 28 foot 4 seasons trailer under 6500 pounds will be a challenge.

Something large enough to warrant 2 AC units, you mentioned under 28', heavily insulated, and an overall weight under 6500 will be almost impossible, especially if the 6500 is a GVWR.
Well insulated requires thicker walls, floors, and ceilings, all add weight, second AC does as well, and trying to stay under 6500 lbs
Outdoors rv models start at around 6000 lbs empty, and thats a backcountry 20' box.
A Keystone cougar like this may work 22RBS but it starts at 5600 empty, so pack light. And I dont know if a 50 amp service and second AC is an option on that unit because it is so small.

To go lighter, insulation etc will drop. You can put insulation bubble material in windows to help, air pillows in skylights will too. But without adding weight , there is only so much you can do to improve insulation.

Sounds like you are more interested in a 3 season TT, which is good, because real 4 season TT, capable of living comfortably well below freezing, really do not exist. Even at 20F, you need to be concerned about pipes freezing.

1. Lance
2. Outdoors RV
3. Grand Designs

Our 2020 Outback claims 4season but is basically an enclosed underbelly with ductwork supplying heat from furnace.
We opted out of 2nd AC.