Newbie to newbie, first define the intended use, are you chasing the sun or weathering winter elements? That will clue you in to the insulation required. You might consider what goes in there and the arrangements before turning the trailer into an ice cooler as insulation takes up space.
IMHO, insulation is overly stressed, consider the heat source and cost of operation along with the fuels required. Not saying don't insulate but the biggest area of concern is first to the roof, then walls and lastly the floor.
Depending on locations and seasons, without extremes, you can get by in 20 degree weather in a tin can if you have the btu to throw out the heat.....and how often is that needed? You might as well be tent camping, which is done all the time.
Cooling is another matter, under the sun, you can shade the trailer with canvas or tarp to keep the sun from turning your roof in to an oven. Air circulation between the canvas and trailer roof can make a big difference. Awnings can do wonders for the walls on sunny days. Insulation will help much more when it comes to air conditioning.
Other considerations are at play, are you going solar, any roof racks with a kayak up there?
Drop back and define the use, where will you be when? What weather do you expect? What is the heat source?
Not to pound on someone else's suggestion, which is about diesel, which can be very nice when you have diesel, one of the cons is that it really smells. You most likely don't want to mess with diesel jerry cans inside a cargo trailer, if you spill that stuff it will stink until next year!
Lastly, you mentioned 3 of you, a 12 footer will be pretty tight for 3 adult sized humans, two is tight and one is just fine, depends on what you can deal with I guess. Think about rainy days, storms most of the day, using the toilet, trying to cook for 3.....you get the picture. I have a 6 x 12 with a V nose, for 2 of us and a boxer, it can be tight quarters.
Best of luck with your plan and be safe!
