Years ago I made the comment here in the forum that camper manufactures removed the nose window do to it being soo prone to leaking and rotting the wood and folks here thought I was an idiot.
We had a 69 Franklin with front window. When the wings gave out from rot around 94, my father tore it apart saving what he could. The only water damage around the front window was from the condensation running down on the inside and that was minimal. I don't lay in bed to look out and often camp below freezing, so would rather have the better insulation than the view. But totally understanding you wanting to keep it.
One thing about the foam board like the blue foam at Lowes is it is a closed cell so won't absorb water. This means any water that gets in, will run out quicker. Fibergrlass loses it's insulation property when it gets wet, takes a long time to dry out, but never regains it's insulation property. Plus, when snugly fit, the foam board adds a lot of strength.