ok.........since i am an "old timer" and have done this, lets lay it out in simple terms SO YOU DONT SCREW ANYTHING UP!!!!
1. remover the vinyl................now how much of a ghost of the old vinyl........... do you have? it looks like a red trailer.......so iam gona say its gona be a lot))))
2. now would be the time to talk to the owner..........and see if he want to have the trailer REPAINTED, because the ghost wont go away any other way.
3. the gray area, if he decides NOT TO REPAINT(it will look like crap if he dont)is best removed with some OLD VERSION EASY-OFF. hard to find today. the new stuff dont work.
4. problem with easy off is THE ORIGINAL PAINT on the trailer. most of these are painted with CHEAP enamels without hardeners. NOW THESE PAINTS WILL BE EATEN BY THE EASY OFF. so
5.i would do some 240-320 autobody sand
paper on the gray..........this way you can take it off till the red starts to show then keep working till the gray is all gone.
6. incorporate something in the NEW VINYL design.......that will cover the gray area.