OP - glad you found the problem before wandering off into the sunrise
I've considered replacing those cross braces with some 1/4x2x2 angle iron and heavier screws plus check the tank supports (and install a couple new drain valves that work easier).
Since I don't have a layout to verify how the frame was designed and built, feel that those cross braces, besides being support fot the tanks may also serve to help maintain the frame alignment vertically instead of flexing at the bottom. That flexing may actually be the failure mode if those screws aren't tight, being virtually cut off in shear mode.
While it may appear that the fasteners have rusted, those screws are very resistant to corosion if our last 5er is any indicator. That unit was a used one but was bought new in Colorado and used in SD plus had yearly winter trips South on salted roads. The screws had some surface rust but were completely reuseable though were replaced by one size larger if for no better reason then I could.