I don't have the answer for ya but I would think it's best to get it off before it gets melty and turns into less manageable ice. It also would initially depend on what variety of snow - wet heavy stuff? or light
powdery stuff? or somewhere in between? Water is fairly dense/heavy, and snow is water, so when it piles up it would make for a hefty load up there.
As far as snow being good insulation that's true when it's on the ground, or on house roofs where you have a cold space (attic) and/or other very good insulation under the roof material itself, but on a fiberglass trailer the roof's pretty thin so I would expect your heater inside would just send the heat thru the roof and melt your snow causing water to run off only to freeze into ice on the sides of the trailer.
Anyway, what I really replied for is to recommend what I use for clearing off snow - the sno-brum
; scratch free sweeping. I use it on my vehicles and I would imagine it'd work well on trailer roofs if you add a longer handle and didn't let the snow pile up too much. There's another brand that comes in orange foam but I can't remember the name of that one right off. This one (sno-brum) I have myself and can recommend it as being durable and worth the money.
If anyone does go the portable shed route most
of those aren't designed for much in the way of snow load either so be sure to knock the snow off the tarp/roof after it accumulates a few inches.