For a quick and dirty answer, how about just setting the dinette up in "bed" mode? Then add a couple of bolsters like these as needed to make it more like a sofa:
JC Penny bolstersThey don't sell covers for those, you have to make your own or find a sewing person to make some washable slip covers. Pretty simple though. We did that but on the opposite side of a mini MH. We kept the dinette but built a box "bed" where there was a sofa we didn't care for. Then the bolsters make it usable as a sofa during the day time.
