I know what you mean about trading in what you have. We would like bigger but after reading the horror stories on here, appreciate our problem free rig.
The thing that gets the coldest in our 3 season is the window frames. Covering those with heavy bath towels made a world of difference. Stuffing the vents and putting rugs on the floor helped too. We don't feel the drafts from the windows and the furnace runs much less. We are use to below freezing at night but don't do freezing day and night so it's not worth the extra cost and weight of a 4 season.