Here are some random thoughts for you. Some are features, some are related to build quality and attention to detail:
Heat duct in bathroom.
Larger duct on longer runs. For example if the blower is in the kitchen with 3" tubes and there is a duct in the bedroom with a small tube (pull the cover to see the tube size) then it simply will not work. All the air will flow to to the larger tubs and the BR will be cold, especially with the door closed.
Space for extra batteries.
Do they use an indoor rated metal electrical box for the wiring on the tongue (which WILL rust in a few years)?
Is there a battery disconnect?
Do the welds on the frame look good - a smooth bead of the same size for the entire weld?
Is the frame already starting to rust?
Make sure axles are not under rated for trailer GVWR. Same for tires.
For cold use you probably want 30Lb propane tanks. Will you be able to change them out, or does it come with 30LB already.
Look under the kitchen and in the bath for holes through the floor for plumbing. Is the hole sealed up (a cold weather unit should be...).
Take photos of the stickers on units you like - see what the as shipped tongue weight is.
Do you want to be able to put bicycles inside, skies, etc. Note where you would put them and if there's room to get them in the door.
Look at the battery wires - are the connections nice closed end connectors with heat shrink to keep them sealed up?
Are the connectors TIGHT - not loose on the wire.
Windows - double or single pane?
Vents - for dry camping you want a couple good quality vents (Fantastic, MaxxAir, etc).
Electric heat source (for winter camping with electric).
Check fit an finish - do the doors open and close well. Do the latches keep the doors and drawers shut with some effort required to open them?
Can you wash your face in the bathroom without banging your head?
