Okay, bit of weird one!
But weird happens, so we need to look for a common item that would kill both coach and chassis power. That one thing is the ground as far as I can think of right now.
Do you have on hand any testing equipment that would show voltage of the batteries?
Assuming you may not, taking a look is about all you have left. Down and kind of below/behind the batteries there should be a buss bar attached to the frame of the RV. That has both the coach and chassis bettery negative/ground connections! You can follow the cables from either battery negative post down to the bar!
Click this snip for better view!
My best guess would be that the bar itself may have come loose from the frame. Since both chassis and coach are failed, this is one point which being open would open the path for both sets of batteries to work!
Possibly it was bolted on and they have dropped off?
The two systems are pretty well independant of each as fully functioning setups, able to work alone. So when both have failed, the one point I think of as common is this ground!
Point to keep in mind if stuck along the road? If you have something heavy duty like a jumper cable, running it from one of the battery negative to the a clean spot on the frame may get it going?
At least enough to get to somewhere better!
EDIT:
Looking closer, this bar may be different than what I thought. See the left wire in this drawing? It shows going to a ground bolt??? Make sure that other end is solid to ground as maybe that single wire came loose???
