Just to illustrate how very durable these 25mm thick sheets of foil faced closed cell foam is, over the past 6 years I have been using a quite large piece of it to use as an insulation between me and the floor, be that smooth concrete, gravel, soil or grass and it is still almost like new, despite having been very extensively used. It is also brilliant as no matter how cold or wet the floor/ground is, I have always remained warm and dry.
So Phil, as if you didn't know already the hump and hollow indentations of your Sister in Laws Sevel van floor won't cause it any problems, even without the additional support of lengthwise wooden runners.
ps. I also keep a smaller section of this magical stuff in the rear storage area of 'Millie' for when I am going under her on site.
Phil