Welcome and nice project. Post some pics when you have it finished. I would think Forest River or a local dealer would be able to point you in he right directtion. I did a quick search and didn't find a lot on that gasket, but it has to be out there
. You can also try PopUpPortal
. They are a great site to locate items for PUP's. lots of restoration work. Someone can help you get what you need. Good luck.