I really have never observed that in my outings... All of my meters are on the DC side... I think it should continue to put out within range 120VAC but don't know that for a fact. They guys that live 24/7 in their big RV's will have to dime in here with that question...
I do know when I am pulling around 20-25AMPs continuous for 5-6 hours I really don't see much dropping in DC Current as my DC VOLTMETER starts dropping down from its 12.5-6VDC to the 12.0VDC which is probably telling me the Power Inverter is still putting out the same 120VAC level.
This is probably more of a battery performance curve than the Power Inverter performance curve for me...
Roy Ken