Perhaps these generalized weights, below, will help you extrapolate your plans:
4X8' -1/8" PLYWOOD sheet 9 pounds
" -1/4" PLYWOOD sheet 18 pounds
" -3/4" STYROFOAM sheet 1.5 pounds
" -1" STYROFOAM sheet 2 pounds
" -2" STYROFOAM sheet 4 pounds
Note: Regardless of what method of building you use, you will need some wood framing. How much depends on your style of building. The method of finishing both your interior and exterior can also depend on the materials you choose and how much you use. So, again, the majority of the weight of your finished trailer will depend largely upon you and the methods/style of building that you choose. A trailer builder can be his/her own worst enemy.