When I run My Honda 2000i, my "plug in" monitor shows "open ground", But everything works. When I run the onboard Onan the monitor reads OK. Could there be some sort of protection built in your rig somewhere that won't allow power if there are any faults ??
Good Luck
2008 GMC 2500HD 4X4 DMax CC LB
2004 Thor Wanderer Wagon 247WTB
2006 TE450 Husky
2007 Rhino Sport Edition
Does the Toy Hauler have an energy management system?
Is there a transfer switch?
I'd check the output of the Yamaha with a polarity tester, and compare that to shore power and to the built in genny.
I'm not aware of any kind of energy management system, and have not seen a transfer switch. I've looked in the onboard generator compartment and also on the fuse panel ...
I have also put my electical tester to the portable generator and it does read 110.