A ways back you posted a picture of the view your trailer was in or near. I noticed it seemed to be a open area.
One of my concerns is wind and if you are in a sheltered area or out in the open.
A berm or skirt is very important IMO as the wind shear is across all surfaces. The bottom having the tanks and much of the plumbing drainage, etc. A sheltered area of evergreen trees, shrubs, etc. can help give a windbreak if blocking the winter prevailing winds.
Another question I have is how is the spring water getting to your trailer and is it flowing in the winter months?
One "trick" to keep pipes from freezing is to keep the faucets running a small volume of water. Moving water doesn't freeze as readily as static water. This movement would only be possible if the spring water was directly connected via plumbing to your tank. Any outside lines would have to be below the frost line or very well insulated.
I think I would opt for heating pads and strips over the lamp.
When covering your windows with the plastic shrink kit. cover the aluminum frame. the aluminum is very conductive to the cold entering the camper. The plastic gives some thermal break here. The joulise (spelling) windows are great for rainy weather, however lousy during winter. The plastic window kit will help. I would actually do the kit on the outside as well. Keep a small bunk window free for venting moisture, or the kitchen if you don't have the bunk window.
I have been camping in my T-1550 with shore power and a 1500 watt ceramic heater and with just one small bunk window slightly cracked open. I have had no vapor problems. I'm not in the trailer 24/7 however nor cooking, showering, etc.. Just during the night hours sleeping. Temps down in the high 20's. I'm not using nor storing any water though. I'll have to set a couple small dishes of water in a few places and see if they freeze. I also have an aluminum underbelly on my trailer. I don't know if it is insulated and if so how well.
You may think about Storm King plastic sheeting to cover the straw skirting. It will help keep the straw dry as well as help with thermal break and finally giving added protection from infiltration.
Keep at it....as well as we all here will. We are with you....well you know what I mean!
Oh. When looking for online answers. Instead of "winterizing trailer", try searching "winter camping in TT" or along those lines. It may provide some useful info. See link below.