Hi there, welcome to the site!
In regards to the mattress you have two options for an innerspring...
1) Direct from Jayco
2) Custom made from the like of places like this
http://www.madetomeasuremattresses.com.au/mattresses/caravan/
I had one in my previous van for our bed that was ordered direct through Jayco when we bought the van new. I found it was to thin and there was a zipper near the end of where you had to fold a section of the mattress up and this often meant the mattress was unstable and sleeping on it was uncomfortable as we where rolling around all night. I solved this by putting down a layer of the egg shell foam material and it was much better.
In my new van I have not optioned for the inner spring and will use the foam one with the egg shell foam on top as it is thicker.
I have heard some good reports of the made to measure mattresses but never used one myself.
Let us know how you get on!