When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
You can jack it up on the frame just in front of the rear wheels or just after the front wheels. Some will jack it up right in the middle under the rearend.
As for if the cold will affect the fiberglass, it will depend on many factor like the condition of the frame and the body itself. I live in Canada and i did lift it on many occasions and never had any crack in my body in 18 years.