Kind of in the same spot here. I have a 1984 Class C with 8.75xR16.5. I wanted to put newer tires on it but just like you I couldn't find any so I did some googling and among other results I got the following.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/22282555/print/true.cfm
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/24602330.cfm
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/25140476/print/true.cfm
I hope they will help you too. I already bought 4 brand new 215/85/16 off kijiji for 350. Looking for another pair same size + spare and rims which will also be challenging. Good luck
Paul