I see a couple of issues with the wheelchair ramp as a bed/settee. The ones I see online all have a short side rail that could cause discomfort or pain while getting on and off the bed. And when folded, the back of the ramp doesn't exactly look flat. I think sitting on it would be very uncomfortable without modifications to the ramp or adding plywood backing to your cushions.
My advice is keep it simple and keep it lightweight as possible. Go ahead and assume you will leave this bed/settee in place most of the time. If you actually need to sleep in the camper incognito, having to set up the bed platform with the top down might be a nuisance. It makes a great storage area underneath for totes and it's nice to have a comfortable place to sit. Access to any electrical, turnbuckles, or built in storage cabinet underneath should be simple enough if you keep the platform lightweight and removable.
I finally replaced the old FWC cushions with high density upholstery foam from Amazon that I covered myself. You can order different lengths, widths, and firmness. I chose firm foam (44ILD), 4 inches thick, 24 inches wide, and 72 inches long. You can buy pre-made cushions and cushion covers, but size choices are limited. You can also order custom upholstered cushions online if you can't find any shops locally.