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Welcome to the Bunnings Workshop community @Olivecaravan24. It's marvellous to have you join us, and many thanks for your question about installing wall panelling in a caravan.
It's great to see one of our knowledgeable members, @JacobZ95, is already on the case. I suspect they are likely correct, and button-head metal screws could be used for the fixings. They are self-drilling, so there is no need to pre-drill the metal. As suggested, seeing some images of the space would be great. Do you know how the ply was previously installed? Let me know if you need a hand uploading images.
Let me mention our caravan renovation expert @twocutekelpies to see if they have any thoughts. If you haven't already, please do check out this wonderful three-part guide: How to renovate a caravan.
Please let me know if you have any questions.
Mitchell
A great selling point for caravans is screws vs rivets in walls, screws usually indicate a timber frame, rivets indicate an aluminium/metal frame. In my renos, I follow old school methods, the ply was originally riveted to the frame, no glue, and the ceiling glued and stapled to the ceiling "battens".
3mm wet panels joiners are great for covering joins, corners etc, although not as great as they used to be since they've made them smaller and shallower, less room for error. Variety of joiners available here - https://www.bunnings.com.au/search/products?q=wall%20art%20joiner
Before installing any panelling, I use a bit of electrical tape/painters tape to locate my frame top and bottom so I know where to drill and rivet, a bit on the floor, a bit on the ceiling. Same for the ceiling, a bit either side on the wall so that I know I'm going to hit timber. Also make sure your panels join on top of frame so the join has support.
I use a narrow crown stapler or brad nailer as well as glue to put my ceiling up, I only use the nails/staples where the overhead cupboards will hide them.
I hate painting so I've used poly coated caravan ply, available through Special Orders from ForestOne (Decor Overlay Plywood), or if you're near The Bunker, that's where I get mine.
As a side note, before you line your walls, think about adding some timber noggins to fit your overhead cupboards to, originally they were stapled/screwed in from outside before the cladding was put on, so adding noggins gives extra backing to screw the cupboard frame into.
Hope this helps. Good luck in your project and feel free to ask any questions. 😊
Shelley
Hi @krumble9
As Jacob said, photos would be helpful. From my experience, the roof frame, including already having the ceiling sheets attached to it, sits on top of the wall frame and is fastened from above before the roof cladding goes on. My caravan ('72 York), in the photos above, had timber either side of the roof with the rafters screwed to it from outside, the ceiling sheets were originally sandwiched between the wall and top.
My 1977 Millard had a similar style roof and I had to replace a rotten section, because these were built from the inside out, I had to replace the timber from the outside (trims/j rails taken off to reseal). You can see the holes in the new timber (meranti) where I screwed it down to the wall frame. I don't have any photos of screwing the new rafters into it unfortunately but hopefully this is enough to give an idea. 
The rafters are only attached to the frame, they are not attached to the above cladding to allow for flex while travelling.
Good luck and happy to answer any more questions
Shelley
