For DIY campervan builders, the first step is the need to visualise how the interior layout is going to be. The old school method is to use cardboard mock up to give a sense of the actual dimension of the fittings. This no doubt is the best method, but not the easiest, especially if you want to explore several alternatives that are radically different from each other.
There are 3d modelling tools around, one of the best and free one is Sketchup. Originally a Google product, it was taken over by Trimble, a name more well known in the GPS circle.