Yes, I have a very good fix. I didnt take any pics of the whole process but here is This link has one of it completed and one of the foam I used.
The foam is 2 1/2 inches thick.
First cut out the black cover under the seat, this will expose the banding straps and the rest of the cushion. There is a center wooden support in the middle of the seat, just cut two peaces of foam the same size as the cavities on either side of the support. Dont cut out the banding, just slide the new foam in between the banding so it fits nicely in the cavity on both sides of the support. The sliding part may require more than two hands. Once the foam is in place take some 1/8" plywood and cut it the size of the bottom of the seat and screw it in place, this will help support the new foam.
When I did mine I just used the thin plywood under the bottom bunk and replaced that plywood with some 1/2" plywood and some hinges since I tend to use that area for storage and I need to get in there to drain the water tank.
A picture of the support is in the link above as well.
One more thing you may really want to consider. This is totally unrelated from cushion thing. Right now I am dealing with a warranty issue that I have yet to have resolved but I may be able to save you a lot of headache. I noticed recently that the cavity where my slideout comes through at the back of the trailer is cracking on the two lower corners. I suspect there is structural damage causing the cracking which is bein g caused by the weight of the slide out when it is retracted and bumpy roads. I would suggest buying some sort of adjustable support to place between the floor and the bed. I used a cargo bar. (See the link above)
Hope it all works lout for you.
Let me know if you have anymore questions.