I did essentially the same insulation treatment to my G20 and while it's worked out pretty well as far as heat control goes, I've found that I occasionally get condensation on the underside of the roof, above the foam and have had to cut a couple vents into it.Painting this is not in my immediate plans. Potentially in the future.
Almost finished insulating the roof as well as well as creating a somewhat flat surface to install the headliner boards to. Discovered the roof is pretty much exactly 3 widths of 48" wide plywood in length and just shy of 60" wide. That is 1" thick, aluminum faced foam insulation board with a R6 insulation value. 1-1/3 sheet covered the roof, leaving me with plenty left of the 3 sheets I bought to insulate the walls with. I left enough room to run the new wiring down the sides for the interior stuff. I will most likely use some of the butyl sound deadner stuff I have left to finish the outside edges as well as some of the weirdly shaped areas. The next step is to finish the flooring, then the headliner. Once those are in place, will create new door panels and figure out the walls. I kept all the old stuff I pulled out for templates.
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Granted, my front doors will allow in some water if the rain is blown just right (even with new weather-stripping ) so I think that's the root cause of it. Water gets into the cabin, evaporates with the sun, and then condenses on the cool metal of the roof.
I recently tossed a couple 100g silica gel packets up there and that seems to have dried things out pretty good.