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.