I forget who makes the Onan portable GENERATOR (Cummins who owns Onan, has it Mfg for them, not Mfg in the Onan factory)
But the EMS should not be needed
The EMS protects against shore power outlets that are wired wrong, an open ground or neutral, utility power transient spike voltages, brown out low voltage that could damage appliances like the air conditioner or 120v fridge, etc..
Your Onan will shut down if overloaded, (does not create low voltage unless overloaded) and does not create transient spikes, won't have an open neutral or ground unless something comes loose due to vibration or damage