Making the dinette into a bed in my TT is extremely frustrating too.
It takes some creativity using the cushions as well as back rests in order to make a full size bed. It's further complicated with where one cushion ends and another begins when that spot also happens to correspond with where your butt rests while lying down. This requires placing the varying width cushions just right so a a large cushion is under one's rump while lying down and as we're all aware, different height people have different rump locations.
I did see one couple with a smaller trailer that had a topper made that rolled up during the day and when the bed was made up the topper just rolled out and provided some bridging at the cushion junctions.
My Son, who is 6' 4" tall found this issue on our first trip together. Now I just let him make up the bed using any combination of cushions he wants. If he complains I'll just bring a tent for him next time.