You may not want to do much hand scrubbing, but that's exactly what's required. I would not use a pressure washer for reasons mentioned above. I can't imagine not cleaning my rig for 3 yrs, but everyone is different. You probably have your work cut out for you now.
I'd spray a small area at a time with rv black streak remover, scrub that in with warm dish detergent in a pale of water, then rinse off before the black streak remover dries. Be careful with it though, it can stain paint.
Then when done, follow it up with a good coat of wax. Our just let it go as you have the past 3 yrs. To each their own. Like anything else, maintaining equipment takes effort, pain and simple.
Your choice if you want to do it right or not. My bil has never waxed his rig and it shows. My 28 yr old rig looks better than his that's 11yrs old, but that's his choice.
