I was planning on using our trailer this winter (but plans have changed.) I looked into having a canvas shop make a skirting with pockets to hold the styrofoam insulation and that would attach with dome fasteners around the top of the black belly wrap. It wasn't too expensive, but would look good, avoid tape and be reusable. It could also be used for winter camping without taking the styrofoam panels. The skirting has two functions: 1. Insulate the underfloor space and 2. stop the wind from taking the heat out from the floor.
If you use styrofoam by itself you can avoid using tape and also prevent the panels from flying away by rigging a taut rope between opposing panels with a small plywood plate on each panel so the knotted rope won't pull through the styrofoam.
Also, make a wooden curb around the ground following the outline of the trailer and use long eaves trough nails to fasten it into the ground to secure the curb. This is so the panel won't pull in at the bottom when you tighten the ropes. The top of the panel will rest agains the AS. Use house wrap tap to cover the seams of the styrofoam. The hardest part will be around the corners, but you could just make them square and put a triangular gusset in the top (carved to fit the gap.)
You could also use some foam weather strips adhered to the panels top and bottom to make an even tighter seal.
If you have electric power a couple of 100 watt incandescent light bulbs will help to keep the temperature under the trailer above freezing.
Humidity will be a major concern inside so you might want to invest in a dehumidifier too.
In anticipation of winter camping I also added a Dickinson Newport propane fireplace
. This unit draws in combustion air from outside and the double walled chimney. This is better than the catalytic type propane heaters which add a large amount of water vapour (and deplete the oxygen) in the trailer.
Good luck - it will be a good adventure.