Easy-Off oven cleaner will usually remove lettering enamels. It may take some scrubbing. It will also dull the factory paint on the vehicle. Easy-Off is basically sodium hydroxide—lye.
I've had the most success using Easy-Off to remove lettering from white vehicles and the least success on red and brown vehicles. I have also gently rubbed lettering off with BonAmi bar soap while flooding the area with a garden hose (it took a long time). Either way, the problem is bringing back the factory finish after removing the lettering. I wasn't always able to do it.
There's no way of knowing how tough the factory finish is. These aluminum skins are coil-coated (roller coated) in huge quantities with anything from acrylics and urethanes to epoxies and polyesters.
I learned the hard way that there was absolutely no way of guaranteeing removal of the old lettering without damage. I have also, more than once, repainted truck doors (with acrylic enamel back then), for people who did not mind a less than perfect job and knew I was cheaper than a body shop.
Obviously, the best course on this job is for the customer to have a body shop remove the old paint and/or repaint the trailer skin, partially or completely, as OP said. And, as he mentioned, on a red background, the lettering will still be visible where the vinyl graphics are pulled off.
Applying new graphics to effectively cover the old areas is a good idea. Or, a large color panel could be installed to the sides, whether vinyl or paint.
If the marbleized panels on the trailer in the photo were painted with urethanes, Easy-Off will probably not do too well at removing them.
Had I done this job originally, I would have marbleized vinyl squares and installed them, rather than marbleizing directly to the vehicle. You have greater control over the marbleizing technique, you can do a lot better job, if you do it on a horizontal surface rather than a vertical surface. I've used this technique many times on flat signs to make a marbleized oval or rectangle.
Good luck.
Brad in Kansas City