I might be speaking of things I am not an expert about but I am going to relay what my experience was. When I had to replace the original bag seal on my older 2003 model. There were obvious signs of significant water penetration into the insides of the bag and down along the two vertical sides of the bag. Every screw in that area was completely rusted out and all the wood inserts that the screws go into rotted away as well. The foam was still there but it was a surprisingly thin and soft material.
So my opinion is that the main objective to prevent water damage is not the material inside the bag. It is finding a good, long lasting outer bag material that will not degrade and allow water penetration into the inside of the bags. Including thoroughly sealing all the edges and attachment points for the outer bag that might be exposed to rain, whether that be staples, screws or whatever.
The main job of the inside foam material is to help form a seal between the upper and lower shells so that rain does not run along the roof into the inside of the camper.
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