by now you may have your slider-door insulation figured out and rolling down the road... just wanted to share what we did on our slider. Here's a picture:
From the inside of the sheet metal we filled all the voids with a combination of shipping bubble wrap + reflectix bubble foil. Then we covered it all with a final layer of reflectix, then installed a layer of hull-liner marine grade carpet and some lower fabric to avoid any rub from the fender flare. So far, three years in & all is working as designed.
The reflectix + bubble foil does an amazing job at sound deadening also. We can be next to a roaring river by day, climb into the van for a nite's rest, and honestly hear only silence inside.