At this point I think you're right about opening up the belly.We always leave the gray water tank valve open while camping. A couple of days before I need to dump the black tank I shut the gray tank valve to let the gray tank fill. Then I use that water to flush out the hose after dumping black tank.
I would be very careful about overfilling that gray tank that indicates 2/3 full. If you overfill it, you could rupture the tank.
The only sure way to find out what’s going on here is to open up the belly.