With 1/2" insulation in the floor, your feet are going to get very cold. Don't leave anything on the floor over night if you don't want it to freeze (leave your cabinet doors open where the plumbing is to keep it from freezing and put Reflectix behind any plumbing on the backside of the cabinet to reflect the heat back into the bus interior). As for your windows. Do what they do with older houses... buy the temporary plastic sheeting and turn your single pane windows into "dual pane" windows just for the winter. We spent two winter in the RV in the Smoky Mountains. I just lined the windows in the RV with either foam board insulation or Reflectix. We have tow doors on the RV. The rear one isn't used much except as a very large window so I placed foam board over that one. Reflectix was applied to the interior screen door with Velcro.
What if I just drilled a several small holes every few feet and sprayed in foam? Does anyone have experience with this method?
We did this with the Eagle frame work (2X2 inch square steel tubing). I felt that the foam (Great Foam brand) did not expand like it normally does. I believe that it is because the Great Foam needs air to help it expand. It might work differently for you but that was our experience. I was foaming in a very tight area with very little air.