I did mine probably 2 years ago. I agree on the heat gun and plastic scraper. If you are planing on replacing the decals like for like then take good clear pictures of the graphics now. Make sure you get the full side of the RV in the picture. This will help you to put the new stickers in the correct places.
Next go take the old stickers off now. Once they start cracking they will become very difficult to remove in one piece. It takes a bit of practice. But once you figure out how hot to make the decal they should come off easily.
And last bit of advice. If your trailer is corrugated aluminum and not flat sided pay a professional to install the new one. It's difficult to wrap the stickers over the breaks in the sides. Let someone with some experience do it for you and you'll be a lot happier.