I have a 13' Scamp
and am a seamstress, not for hire. The cushions are simple in design and structure and should be easy for a seamstress/tailor to make. Because the cushions are covered in a heavy weight
fabric an industrial sewing machine would be the best and most home sewers do not have an industrial machine. I suggest contacting a trusted person who does alterations exclusively or associated with a dry cleaner. An upholsterer should be able to do this job. You probably will need to provide the fabric, foam, zipper and pattern. At the very least you will need the pattern. If you have the old cushions you could give the seamstress those. If not, trace and measure carefully the outline of all the cushion spaces using brown kraft paper. If it's convenient you could even drive your trailer up to their door!
Since you may have your cushions made custom you may want to have the cushions made in shorter pieces so you have easier access to the hatches under the seats. What I mean is, instead of one long cushion you could have two cushions. However, this may involve more labor and therefor more cost.
You are actually in a very good position to choose your own foam and fabric so you get a custom look. You may be able to get both at a discount. Ask the seamstresses you are considering for recommendations for discount sources of foam, fabric and zippers. Also ask them to estimate and amount of yardage and the number, length and type of zippers you will need. You may also need to supply thread. If your fabric has any type of repeated print or plaid you will need more yardage. Ask your seamstress how to estimate.
When choosing your foam keep in mind that most people find the factory-installed Scamp cushions too firm for a comfortable sleep (Including me). You might want to get a slightly softer density foam. Do not go too soft, however, as the seat will be too squishy.
See my solution to a good night's sleep at this posting #11 Memory Foam Mattress topper?
and see my photos. I will be glad to consult with you in a PM (private message) if you like.
Personally, I would have preferred softer cushions and I would top them with a cut to size wool fleece mattress topper (and eliminated my foam topper)https://warmthingsonline.com/wool_fleece_mattress_pads
if I had had the choice and the money to do so.
Lastly, I suggest you look up previous postings regarding cushions and bedding on this FGRV forum. Go to the tab at the top of the page that says "Search" go to the bottom of the drop down menu and put in the words "bedding", "cushions", etc. in the Google Search window. Best of luck.