Hi @Amelia461
My interpretation of Airbnb is that you can list any accommodation, as long as guests know exactly what to expect. From your side you should just be confident that you make the issues clear in your listing description and in your photos, and that the price seems reasonable for what they get (otherwise expectations can still be too high).
If guests still do not seem to take this information in before they book, it places the issue in the same category as any other information that you specifically want to bring to their attention. Some hosts have to highlight issues regarding parking, air-conditioning, open pools, or whatever. Your point to highlight is that the caravan is not new and luxurious.
My suggestion would be to come up with an appropriate one-liner to draw guests' attention to it, and then add it in your house rules under "additional rules" and/or insert the one-liner in your first message after receiving an enquiry or booking.
You can find lots of suggestions on how to highlight issues to guests in this recent discussion:
How do you encourage guests to read your house rul... - Airbnb Community (withairbnb.com)