See attached file

1 - in top view, draw the outline of walls, floor and ceiling
2 - add a room with floor, no ceiling
3 - make wall height half the width of your caravan
4 - insert doors and windows, rotated 90 degrees
5 - apply textures etc
6 - export as OBJ file, selection only
7 - import and rotate 90 degrees
8 - copy once, mirror and rotate, then join the halves

Disadvantage:
Walls can be seen from the inside, not from the outside
This could (probably) be solved by exporting just the floors (second time) and applying these to the outside.

Note:
Standard doors and windows can not be inserted in floors. Use your own doors and windows, import them as stairs, and see if that works better.

H