Here is info showing the original layouts. Your should be similar, rear bath, beds in the middle. This will give you an idea of what used to be there. It looks like you have a blank slate.
If the tanks are still there, they will dictate where the bath needs to be but the water tank would have been above the floor across the front and the black tank above the floor under the toilet and sink in the rear and they don't show in the pictures. The grey tank would be below the floor between the frame rails. If they are gone you will have to install new tanks, but can move the bathroom.
Past President, Vintage Airstream Club, WBCCI # 7291
1977 Argosy 28 Center Bath (CA made) and 2015 Ford F250 6.7 diesel. Used to own 1978 Argosy Minuet 6.0 and 2013 Toyota Tundra