I have worked in a sign shop for 14 years applying and removing vinyl. I have noticed many people asking about removing their vinyl so I will give a detailed description on how I do it. I have not done a trailer but I am sure the process would be the same as my signs.
First off, there is no product that you can put on top of your vinyl to make it come off easier other that plain old heat. We've tried stuff, it didn't work!
Try this in a small hidden area first. With vinyl the batches varied from day to day it seems. It can come of in decent sized chunks or tiny little flecks. If yours is looking like its doing the tiny fleck thing, abort misson and try something else.
You will need a :
Blowdryer or heat gun, Exact-o knife, Glue remover ( Rapid Remover, Goo be gone, Goof off ect...) Spray bottle, Semi flexible plastic scraper, a lot of paper towels and a garbage can. Mister Clean Magic Sponge and 000 steel wool is optional but handy if you've got glue around rivets.
Make a slit with the knife in the vinyl near a natural seam in the trailer. If you do it in the middle of the panel the cut line will be visible in your final finish.
Heat a small area up with your blowdryer then carefully catch an edge of the vinyl with your exacto knife held at an angle and peel back slowly. I set the heat down while I'm working on peeling but I have heard of people setting it up on a tripod and moving it along as needed. I do not recommend a straight razor for removal as the straight edge is guaranteed to scrape the round trailer.
Next you will have the old glue to remove. I use Rapid Remover that we purchase from our local sign supply place, I also use the scrapers that are used to apply vinyl to scrape the goo off so pick up a couple if you do go to a sign place. There are many other products that work almost as well as Rapid Remover. Just read the labels and look for something citrus based. If it does not come in a spray bottle transfer it over to one. You can buy it by the gallon at a sign supply place and you may need it, it's about 35 bucks but way cheaper in the long run than buying a bunch of tiny bottles.
For the initial test in the hidden area do get a small bottle of your product of choice first to make sure it does work on your adhesive.
Protect your work area. The glue remover sprays will make some plastics go tacky so cover them carefully. Work from the top down in small one foot patches. Spray some onto the area and let sit for about 45 seconds until it looks slimy. If an area looks dry, spritz a little more on that area. Catch the spray running down the wall with paper towels. Scrape the goo from the top down into the paper towels and repeat if nessesary. Do not scrape to soon or it will not work.
Be warned the softened glue looks like boogers and slimes like boogers. Have your garbage can very near by and easily accessible. It may be a good idea to take your fabrics out of the trailer.
For the final bits of glue residue saturate a paper towel and rub the area down. This is where you could use a Mr Clean magic sponges or 000 steel wool to get the glue around the rivets. The magic sponges only work while wet so if its not working anymore add more glue remover to it.
The rapid remover can be used on glass as well if you have old decals you want to remove. It will also take off grease and tar spots from the outside.
Your trailer will smell like an orange juice factory after. Open the doors and fans while working and it will eventually dissipate.
Hopefully this will help answer some questions.