First of all Charlie, I would put the cinder blocking back under the original perimeter wall that they took out. At least for support as all exterior walls are load bearing. Then I would suggest running batted insulation (in the addition) between the joists, especially if you have any plumbing running under the addition. As far as heating, I know skirting of some kind really helps and if you have skirting around the entire trailer then you could heat underneath and not lose heat. I have never lived as far north as you are, but heating underneath would seem to be a real good thing to do. Hope this helps.