While digging into the underseat storage on my primary dinette seat the other day, I discovered that the front panel was trying to escape from where it had been nailed to the sides. After gently hammering it back into place, I went to Lowe's to find a solution.
This ought to do the trick...
Under $5 for the pack of four, and yes, I did add the missing screw.
I just completed this same project on my T139FK. We use the seats as twin beds and feel better about the strength. Also used "T" straps at all cross pieces. Next up is using 1x2 material for extra support underneath the bed (seat) frame ...
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2005 T139 (traded)
2011 Gulf Stream 16BH
2010 Ford Explorer