RESOLUTION!! Today I had a chance to figure out my buddy's electrical problem. I had VOLTAGE from the battery to the fuse board. And voltage from the converter to the fuse board. But nothing would run on the battery. DUH, I should have thought of corrosion. There can be potential thru corrosion, but as soon as LOAD is applied, the high resistance of the corrosion causes a high voltage drop and thus nothing works. DUH. So I traced the wiring back to the battery and there it was....a 20 amp fuse shoved beside the battery near the bottom and all corroded. Neither my buddy or I pulled the excess wire out of the battery box or we would have seen it. I replaced the fuse holder and added a battery disconnect for him.
Just a refresher, just because one has 12 v on a circuit, doesn't mean current can flow.
Thanks for everyone's help.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.