Simple Green does come in different strength levels. For the very concentrated kind, I find that you have to either order it or buy it from a speacialty store. It will strip paint and cut through just about any stain. We used it, and they still do, in the USMC along with CitriClean (stronger than Simple Green). I used to order it in 5 gal. buckets and in barrels for after deployment cleanups for 2nd AA Bn. As for what you see mostly in stores, that is a much milder and I've used to strip decals off of my company's vans while not stripping the paint. I used it with some other speacialty chemicals, but be careful by rinsing quickly after use.
I also heard that Zoom and Bam work wonderfully. I haven't tried them myself, but people on RV.NET swear by it.