Here's some more info on what I've tried as of now.
My receiver is an Onkyo SR505, the remote being the RC-681M. I've tried the remote codes for funai, orion, vizio, westinghouse, aoc, envision, memorex,
Mitsubishi, Broksonic, Sansui, Insignia, Durabrand, and emerson. Several of the codes will allow my remote to turn the tv on and off, and adjust the volume, but I can't switch inputs, so that's still essentially useless.
Has anyone gotten one of the Harmony remotes or something similar to work with this set? I'm spoiled on only having one remote for everything.