Rebecca,
How much traveling do think you would be doing in your camper? If you will be be fairly static, just moving once in awhile for short distances at relatively low speeds, what you are envisioning may be doable. You still need to keep the gross weight within the chassis' and tires' specifications.
If you are going to be moving around frequently and/or over long distances at highway speeds, I would go for the lightest solution, even if that means using modern materials. You're in a van, not a cob cottage. You won't be doing the earth, or your pocketbook, any favors lugging around hundreds of extra pounds. You can always give it a funky earthy/rustic veneer.
Have you looked at cotton or wool batt as an insulating option? Closed-cell foam and/or XPS panels are probably the most efficient option. Aluminized bubble insulation can also be very effective if it is installed correctly, especially in conjunction with some kind of batt insulation.
I have no
experience with straw-clay, but I have researched it. You can probably get a fairly lightweight, consistent result with some trial and error and lots of practice.
I looked up marine/small stoves. There are a some very nice products available. They aren't particularly cheap. You might be able to look around and find a good used one for a reasonable price. Another option is a tent stove. Quite a variety, but they aren't as pretty.
Good-luck, and remember that just because something is natural doesn't mean it is healthy, or vice versa.