I'm not really familiar with FHEM, but as far as the MySensors side, you still need the gateway software running. However, this code can run directly on a Raspberry Pi, at least with a radio network for the MySensors transport. I haven't ever used RS485 myself, but I would think it would be similar.
At least the code itself has sections for dealing with running on Linux, which is what is used when running it on the RPi, so I think you're good. https://github.com/mysensors/MySensors/blob/master/hal/transport/RS485/MyTransportRS485.cpp
The gateway code runs as a systemctl service, so it can easily run in parallel with most things, so probably wouldn't conflict with FHEM on the same device.
Oh yeah, and in case you haven't found it, there's this page: https://www.mysensors.org/build/rs485