Eco-Mode was blocking the Champion from working!
Did another test with Eco-Mode off -> Chargerverter on the Duel fuel Champion 2500-Watt (Model 200961) -
https://www.amazon.com/gp/product/B08L45W2V9/ref=ppx_yo_dt_b_search_asin_title
View attachment 206373
and it worked. You have to explicitly turn Eco-Mode off. The advertising says "....enjoy 2500 starting watts, 1850 running watts..." but NO WAY did I get 1800w.
16a was the max current I could set at the Chargeverter. 17a and the Champion started bogging down.
16a to 53.8v battery = 860.8w of charging. The kilometer on the Champion showed 947w of AC going into the Chargeverter.
860.0w/947w = 90.9% efficiency for the Chargeverter.
The Champion works (good) but it can't do 1850w (disappointing). More like 1000w continuous on gasoline.
Note: The MaXpeedingrods 3500 had eco-mode on as well, and advertises "...With 3500 peak watts and 3000 running watts, ...". I did 50a (e.g. 50a @ 54v battery = 2700w of charging) but it was disrupted when MPP Solar did a large load on the battery so maybe eco-mode off will solve this as well. At a 90% efficient Chargeverter then that's 3000w of 120v generator AC -> 27000w of charging and that's close enough to my rose colored glasses expectations and I'm happy again