You do not have to decide everything before starting. Gather a few details you can check when measuring the van or taking your first trip.
Define your use before drawing furniture
Note how many people will travel and sleep in the van, and whether you will take a dog, bikes or other bulky equipment. Decide between short trips with frequent access to services and several days off-grid. This affects the bed, walkways, storage, and water and power requirements.
Measure what determines your layout
Use usable interior dimensions, allowing for lining and wheel arches. Check doors, working height and areas that must remain accessible. Save the vehicle generation, length and height; a layout seen in another van is a reference, not an exact dimensioned template.
Start with one scenario and compare it
Enter realistic consumption and autonomy requirements, draft an initial materials list and compare a lighter or simpler alternative. Keep the date and assumptions for each option. Whenever you change a tank, battery or item of furniture, check weight, space and budget again.
Continue on another device
Local saves belong to that browser. To move a project to another device, download a project backup using the available options and restore it there. Keep a backup before replacing a configuration with another proposal, too.