Xvid4PSP has support for RMVB input and has a couple of presets for Ipod mp4 output. You can adjust the settings of course. It is free to download and use, so worth a shot.
Take note of the minimal requirements, which include Microsoft Framework 3.0 (there is a direct link at their site if you don’t already have this installed).
If you try it, I’d appreciate some feedback, as I don’t run into RMVB files.