Are you sure that it is the support of the platform or is the mattress shot?
We spend several thousand dollars for our beds in our houses. These RV mattresses are $200 items. They don't last long.
With no bedding on the mattress can you see two indentations in the mattress? If so I suspect it is time for a replacement.
You could always try to add some support under the platform if you suspect that is the problem.
Peace
Dave