The exterior wall that the box is mounted on is extremely flexible. I can easily push it in three to four inches, and it will quickly spring back. I believe that this flexibility caused the box to crack. How do I stiffen it as it is not easily accessibly. I am pretty sure that I need to remove the fridge and stove to get to the wall, then what?
I have a similar spot on the wall opposite the door. Most areas are pretty solid but there some areas of unsupported panel. I can think of a couple options, but if the area has a void between two fiberglass panels, some kind of durable polyurethane foam might be a good choice to fill the gap. You want to avoid the stuff that expands aggressively, because it can damage the walls with pressure.
If it's the hatch itself that's flexing you can frame the inside opening to stiffen it, with some marine plywood or a frame of 1x lumber.
Once I have rewired mine I am actually looking for someone to blow foam the whole thing. It should insulate and increase the structural integrity. The only downside I can see is that fishing wires and tubing becomes very difficult, so I would not want to go within 12 inches of the floor with it. Manufacturers that use expanding foam will install some kind of conduit as a cable raceway.
The other options would be spacer blocks glued in place, or you could 'glass in some kind of buttressing to span the unsupported area.
Just my thoughts, not yet put into practice!