You can make them yourself, you can purchse foam from Jo Ann's Fabrics and use the current covers carefully disected as pattern pieces, assuming of course that you have the current cushions. You can do it without too but a bit more tricky, you can best cut foam by the way with an electric carving knife, like you carve a roast with.
Any good upholsterer can do cushions for your Camper. You could do it yourself if you have a good solid sewing machine. Box cushions are not hard to make. You can get the foam cushions online or from a local upholstery supply place.
Look at youtube for a video by Sailrite. https://www.youtube.com/watch?v=DCADkbAqqMQ
It's really quite easy if you have any home sewing skills and room to work and the patience to take your time and do the steps carefully.