I used to own a fifth wheel that had a dandy little wedge cushion the made the back rest of the sofa perfectly comfortable. My LD back cushions, and I imagine other models as well, have a vertical back cushion and I find it offers no lumbar support, so I always ended up tucking a pillow at the small of my back...
Then the light
went off, why not make one of those wedge shaped thingies?
I got a twelve inch high, 6 inch deep, 20 inch wide piece of dense foam, gifted to me...I marked each end with a line that went corner to corner and cut it with my electric bread knife. I covered it with some quilting cotton, just folded the fabric over and stitched by hand. This made two cushions that were six inches deep tapering to nothing, twelve inches high and the width to go across the seat back.
I simply stand the cushion on its wide edge at the base of my back rest then stand the original cushion in front. This makes the back cushion slanted and by golly it is comfortable!
I have my dinette made as a bed always so I use only the two narrow seats that are across from each other by the cabinets. I keep the small table stored when not in use unless I want a chaise..I love this little trailer!
Just wanted to share in case you also need a little extra back support and want something that stays in place discretely.