I have painted four small strips that cover the seam between the front and rear caps and the roof on my MH. Before doing it I called the company and talked with them about it. They said that I could paint it, but it probably wouldn't last due to the slickness of the eternabond surface. They suggested I rough up the surface a bit with sandpaper before painting.
I gave it a try and the paint has adhered well enough that I have not had to do anything else with it in nearly two years. I did not roughen the surface as suggested. I'm careful to not expose the painted surface to any high pressure water or abrasive substances.
As far as your application, I do not believe that the bed of a truck is a friendly environment for painted eternabond. It won't hurt to try it, but I'm pretty confident you will find the paint peeling off in short order.
2005 Newmar KSDP 3910
ISC on a Spartan