I've never grounded my Onan inverter generators. I've never run the EMS with it either. My EMS is portable and the generator is suppose to have self-protection to keep electricity clean, and so far (5 years now), I've not had a single issue with the generators (I have 2 of them I run in parallel).
But ... if you have a fixed EMS in your camper, or a permanent EMS in your camper and it's not removable at whim.... then yes ... you'll need to go through the grounding process for the generator, otherwise your permanent in-line EMS will never allow electricity through!