I thought I had tried everything to remove old dried up expanding foam insulation applied sloppily to all of the cracks in the ensolite on my '81 Scamp
by the previous (bonehead) owner. I was convinced that it couldn't be removed. I have sprayed a dozen products and scrubbed and scraped for hours with little results, UNTIL...
I don't want to sound like a snake oil salesman, and I have no relation to this product, but Motsenbocker's Lift Off Caulk, Silicone and 'Foam Sealant Remover' did the trick. It is a gel that is sprayed on, I let it sit for 3-4 minutes and then I scraped of the previously impervious dried insulation foam with a plastic dinner knife. It took two applications and I got it cleaned off. There is still a bit of scraping but I was pleased with the results.
There is another thread about Motsenbocker's removing silicone caulk here - http://www.fiberglassrv.com/forums/f55/silicon-remover-51074.html