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Update. I managed to get into the rear of the DC breaker panel.

I initially thought i could get in via the front panel. I could tell the large cables on the battery switch were preventing the panel coming out so thought I would undo the screws holding the battery switches in place. Would have been effective if the nuts on the back of the battery switches were captive but they aren't as I learned the hard way. Abandoned this approach.

Next I used a borescope and confirmed there is an access hatch at the rear of the slide out grill.
I removed all the drawers, unscrewed and lifted up the grill, photographed the AC wiring on the relay and then disconnected and removed the grill from top drawer.
After this it was a matter of removing the slide out drawer which required unbolting the 2 x gas struts, unbolting the front stop spigot and then undoing the 3 x screws that hold the drawer to the runners each side. The LHS rear screw is hidden behind another small panel.
Removed the slide out drawer and the DC breaker panel access hatch and I have access to the wiring I need.

To change out a battery switch you would need a second person to undo the 4 x screws on the battery switch from the front panel while the second person catches the nuts via the rear access hatch. Obviously disconnect your batteries before using tools inside this space.

As luck would have it I have identified that the AC cable that supplies the grill has been getting pinched and damaged when the grill is closed so I will fix this before it becomes a bigger problem.

I hope this helps someone.