I’ve found the seat cushion ‘stuffing’ in many dinettes, & much RV furniture in general, to be abysmal. Sometimes it’s not even foam but rather some fibrous material that squashes down in short order.
The good news is a lot of these cushions have zippers so you can replace the cushioning quite easily. We tossed out our junky OEM stuff by way of going to a local upholstery supply place & getting high quality foam. Night / day difference for $60 & 10 minutes of our time zipping in the new stuff.