I have had 2 Claas A's that had fogged windows.
The REPAIR is very expensive and I just would not do it.
I have also had many Class A's that did NOT have the two pane windows, and my current FW only has one pane windows as well.
My conclusion ... they are NOT necessary , unless you are a full-timer in a northern area.
Solution (?) ... If I became the owner of another unit with this problem ( which is quite likely since I buy-sell a lot ) ... I believe I would remove one of the windows.
This may not be a wise solution for a newer unit with a high value, but on anything less than that .. it seems practical since each window represents $1000 or more, from my investigations in the past.
One other idea might be to drill a syringe sized hole in the top of the inside window, and fill the center with Anti-Fogging fluid. It will leak out the bottom, since that is where the gas disappeared to, but the residual effects may provide temporary clear-ivity !
Caution here ... I was told once that introduction of foreign fluids might just turn the window more black ....
Not easy decisions ... difficult problem unless money is abundant.