Yup, heat gun. If the decal is applied on a fiberglass gel coat, be very careful. Very easy to take the clear coat off with the decal. And usually there will be a "shadow" left where the decal was.
The heat gun will be the best idea for this, and some of it will peal off anyways. What would most on here charge for decal removal on a 24ft u-hall truck. Its not everywhere just a bit on the sides, and the entire front and back are covered
Heat gun and rubbing alcohol/lacquer thinner for the glue left behind. Very time consuming for us. Takes two guys for large graphics. 2-4 hours minimum for a U-Haul I'm guessing. If you're unsure ask for time for a test. For stuff like this I always tell them if they want the best price let me try a test. Otherwise I have to assume the worst.
It took me and a helper 3 hours to remove decals from 3 sides of the 8'x20' enclosed trailer using razor blade scrapers but I did it that way because they were very hard to remove and I could not find my heat gun or I would have tried with the heat gun.
I'm in the vinyl "business" myself, and I'd recommend you Rapid Tac Remover. Look for it on YouTube, lots of info and videos.
What I'd do is get in touch with your local sign guy and ask for a quote, and if is less than what you've quoted, sub that out and make some money without headaches, removal is a b**ch