Are you asking how to cut the bevel or how to establish the amount of bevel required along the length of the shelf?
The actual cutting can be achieved by as suggested above, with a jigsaw angled to the smallest angle and then use a block or smoothing plane to fine tune.
As far as marking it out, or establishing the bevel along the shelf, then that can be done by creating station marks at regular intervals along the length and so long as the caravan is set up level, use a spirit level held vertically and touching the top of the shelf line at each station mark and measure the gap between the level and the bulkhead at the depth of the thickness of the ply you’re using, ie 18mm if your using 18mm ply. Transfer each measurement to the underside of your ply shelf that already has the horizontal curve cut. Join the dots and get jiggy and planing. Or you could use the shelf itself, although I imagine it’s going to be tricky to hold the shelf in position for you to do this and mark accurately.
If you need to establish the curve of the shelf, then fix a straight edge horizontal and level along the length of the bulkhead and again, with station marks at regular intervals, measure the distance between the bulkhead and the straight edge and transfer these dims to your bit of ply.
It’s like boatbuilding but won’t need to be as accurate 😀