Your dinette table was constructed with the lowest grade particle board available at the time. Atmospheric moisture and gravity are all it needs to warp. Think about how it's made. There is a moisture proof laminate on the top. There is exposed particle board on the bottom. The exposed particle board absorbs moisture from the air and expands (swells) but the top does not. The bottom surface grows in length and the top doesn't hence the bowl shape of your table.
As others have said and I have done, make a new table out of some 3/4" oak veneer PLYWOOD, not MDF core. Put some 3/4 x 1" or 1.5" solid oak edge banding around it. Next apply 3 coats of Minwax Polyureathane or equivalent to BOTH SIDES. This will eliminate bowing due to humidity and you'll end up with a much nicer, sturdier table to boot.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.