I would also HIGHLY recommend blue shrinkwrap instead of white
Been shrink-warping my boat better part of 20 yrs now and I'd agree with the shrink-warp as being better then canvas for long term winter storage. No doubt, canvas can hold moisture, doesn't shed snow too well and aren't typically big enough to make high tent poles, not for heavy snow loads anyway.
HOWEVER, I do have to dis-agree with using blue wrap though, tried it once and it was a disaster!!
Yes, it tends to warms the air underneath and that's the problem (in my area anyway) as warming the air can/does create a temp gradient ........
If there is a temp gradient (colder air on the outside and warmer air inside for and in our example),then this warmer air which always contains some moisture content (1-99%) is now held/present inside and under the wrap AND if this warmed moist air goes up against the wraps cooler surface (recall it's colder on the outside due to warming on the inside) and also combining that our plastic wrap isn't much of a temp insulator, it will instantly condensate on that surface of the wrap....Think of a glass of ice water sitting on your dinning room table.
The underside wrap surface now becomes a dripping nightmare from hell...........
Inside surface condensation will collect anywhere there is snow laying on the outside and/or when colder air exists on the outside verses inside. Even with high tent pole designs, some snow always still collects during and after storms. And even if it sheds ALL the snow, the blue color now tends to always cause even more of a temp gradient with the slightest bit sun/light.
I found that keeping the inside air as dry (moisture collection bags as most have already noted) and as COLD/EQUAL as possible (lots and lots of vents) to the outside temp works best, and for me, the white wrap helped keep inside/under temp as equal to the outside air temp better then blue did as white wrap causes less solar heating and therefor equals less of a temp gradient......
