MBS spare ignition key
Take my advice and always replace your spare key if you lose it. It will cost you a lot less in the long run.
I lost one MBS ignition key while camping but didn't get a spare. Then I lost the other one at home(!) (Yeah, I know. Dumb, dumb, dumb!) Cost me several hundred dollars for a new key and programming. Only MB can do it when you have no key to copy. Also had to have RV towed to MB dealer for this (thanks Coachnet), but first, unhook the driveshaft for towing. Big hassle, and if it had happened while camping out in the boonies it could have been really bad!
Once I had a real MB key again, a local locksmith was able to CLONE it for $90. So if you can't find your spare, find someone to clone your MB key. You may need to call a few locksmiths to find one that can do it.