While taking a quick look around the net for information about a 40k btu furnace sail switch, I found that there is not a lot of websites that offer the type of support that the Escape Forum does. I imagine we are somewhat blessed that we have the abundance of technical help that we do for our trailers.
That said, I did find several 40k btu furnaces that could be the one the OP is asking about.
https://pdxrvwholesale.com/products/...40-tune-up-kit
http://techsupport.pdxrvwholesale.co...ing-Manual.pdf
https://pdxrvwholesale.com/products/...21-tune-up-kit
https://techsupport.pdxrvwholesale.c...als/afld40121/
Both these links give parts breakdowns and part numbers.
I looked up the owners manuals for Keystone trailers and found that Keystone uses both models for your trailer - you will have to look to see which one it is.
https://www.keystonerv.com/owners/owners-manuals/
Hope this helps -- and if you are ever in the market for a great fiberglass trailer, remember us!