I know that this has been a topic in the past but I would like to add my experience to those of the past. Like others, my wife found the beds in our Troll 540 too hard for her to get a good nights rest, even with 2 toppers! So I took the plunge after asking others on here, and bought some foam replacement. I went for Reflex superior medium after making a visit to Efoam in Wolverhampton. They were extremely helpful and with free delivery I had my foam delivered in just a couple of days. Changing the cushions sprung interior was very simple and on first trip out this year the bed proved much more comfortable for my wife. I can recommend changing if, like my wife, you find the bed too hard.
If you would like to know the measurements I will gladly supply along with the price I paid. After all, with all those activities we get up to on hols, a good nights sleep is essential