How structural are the plywood panels in cabinets?

As it turned out, the plywood was only marginally glued to the 1x2-ish framing (at least they used some sort of harder-than-pine wood); so it will probably have more structural strength after I finish installing the 1/4" plywood cover with suitable screws.

Two hours to lay out and cut the plywood out (by hand); five minutes or less to reattach the PEX to the check valve, once I could reach in with both hands in a comfortable position. Winnebago should have put in that access hatch at the factory.