Sounds like a good test, I'll try it the next time out. But, for the record when we came home last week from Chilliwack, the battery "meter" was at 3/4. When I plugged in to the 120vac the converter was certainly in "high" mode. You could hear it.
I let it charge for well over 24 hours before I shut it down. Latter, I checked the specific gravity , and found it not completely charged. And you only know if a battery is charged by checking the specific gravity. Everything else is an educated guess. The Diehard charger completely charged the batteries.
The on board WFCO charger puts out 13.6v in absorption (normal) mode. But the Interstate Battery spec sheet for the GC2-XHD-UTL clearly states to charge at 15.3v, Even the float voltage is lower than the battery manufacturer recommends.
I'll keep studying this "problem". We tend towards so called boondocking...but I consider it plain old camping in a campground without hookups. Solar is in our future, but I'm trying to get smart first. So far the best info I have found is at:
HandyBob's Blog « Making off grid RV electrical systems work
. Love him or hate him, he has some great insight because of years of real boondocking in a big 5er without owning a generator.