I've seen it on all our previous RV's. I believe it is dew that forms overnight on the exterior and gets scorched, for lack of a better word, by the sun. The temperature differential between the insulated areas and the non insulated studs indicates where the temperature differential is.
We're not too far from the coast and have very heavy dew and then when the intense sun gets up it cooks the dew.
It will cause oxidation of the exterior siding. I fought it for years...polishing and waxing. I suspect I compounded the thin shiny coat off the filon exterior.
I traded it off for the full body paint Landmark.
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2018 Ram 3500 Laramie CC DRW LWB 4X4 Cummins Aisin 3.73
Reese Goosebox 20K
2018 Heartland Landmark 365 Oshkosh
2008 Bigfoot 25C9.4 LB Cabover
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