We had a similar problem on our 2307C - the previous owner had the bottom cushion foam replaced but left the original foam in the backrest, so it was a LOT softer.
We had both cushions replaced with high density foam, and also had a 2" thick full width high density "topper" made and upholstered - it's pretty firm, and very comfortable.\ 95% of the time, it's just two of us using the trailer, so we leave the sofa set up as a bed full time.