I would start with a very high pressure hot water pressure washer. Looks like your truck left the factory tan and then uncle painted it green with the usual zero prep work. Hopefully most of the green will come off. You want to paint over the factory tan. If you resort to chemical paint remover, that will cut into the base tan coat and you will be into the bare aluminum which requires special etching primer. It is very difficult to properly feather edge any remaining thick green with a DA sander so it looks good under top coat, so I would avoid sanding if possible. You are looking at weeks of work.