I am not at all familiar with your rig, but I do have some sewing experience. Most people just recover the futon itself. If you go to a futon specialty store (look near a college campus), you can probably find a new slip cover for the futon itself. If you sew, they aren't hard to make - just a big box pillow style. Pillow cases might work for covering cushions. This is a time when lots of T-pins would be helpful to tailor the fit some.
Sorry can't be more help, but no one else was responding so I thought I would give my limited knowledge.
2014 Outdoors RV Timber Ridge 240 RKS
2015 Ram 2500 Diesel