Leak in 7500 watt Onan Diesel Generator

I know you said it's not the filter but let's back up. Has anything been serviced recently? Are you positive the filter is tight enough, I've seen a lot of loose filters. If it was serviced be sure they didnt double gasket it. I'm sure it's a tight fit for everyting but it would be good to see exactly where it's coming from before you pull it out. If you can run it while it's out that's good. I bought a $15.00 flex snake camera that can connect to a computer or cell phone that might let you sneak in and look closer. Brake cleaner and an compressor with air blow nozzle will dry things up.

Is that rectangular cover riveted on? Is it a possible place where you may be able to see more?