Onan Microquiet 4000 - Ground Issue

You said if you jumper from Generator frame to chassis battery negative or to coach frame no joy but if you jumper from generator frame to coach (house) battery negative joy

Clean house battery connections.. On negative if you can find it clean both ends of that cable and see if that works. You have a poor connection on house negative lead not sure which end.

What part exactly are you considering to be the "generator frame" as opposed to the "generator"? The usual chassis ground connection on these generators (for the 12V system) is from a bolt/stud on one of the corners of the pan part of the generator housing, the inside of which has a sort of star or bottle-cap shaped washer with a few smaller ground wires connected. The starter motor has a ground lead attached to it, but I'm not sure offhand where it connects to the generator chassis; if memory serves, it wasn't along with the bottle cap shaped washer thingy.

If I'm understanding your post properly, it sounds as though the ground connection between the generator and the chassis of the motorhome is not good. These connections need to be solid as the starter motor requires something on the order of 100 amps when cranking.