Foam Factory has all the kinds of foam, including latex, and their prices and quality are excellent.
My suggestion would be to make the seat cushions as comfortable as possible, so that you need only a thin (1") mattress topper, or none at all. If the foam in your seat cushions is in a bad state, replace it with some of the high density stuff.
If they slide around, find a way to secure them.
The reason I suggest this is that you're going to find a 5" mattress topper hard to store, it'll be very bulky.
I replaced the foam in my salon
cushions with 4" of HD50, 1" of HD36 glued on top, and 1" of batting. They're now very comfortable to sit or sleep on.
The V-berth has the same but with 1 inch of softer foam in place of the batting.